The Zerg race is very unique from the Terran and Protoss. The Zerg have mostly weak units, but opperate well in large groups. The most unique things are the use of the Overlord to distribute Control, which is needed to morph new units, the upgrades of the Hatcher to a Lair, a Lair to a Hive, a Spire to a Greater Spire, and a Creep Colony to a Sunken Colony/Spore Colony. Unlike the other races, Zerg have the ability to train many units at once, instead of one at a time. This helps them get many units at once. Making multiple Hatcheries is very useful, whereas it is useless other than expanding for the other races. The Zerg have to build on Creep, wheras the Terran can build on plain land and the Protoss must build near a Pylon. The Protoss and Terran can build many buildings with just one SCV or Probe, but theZerg must use one Drone for each building, often making things hard for them. The ability to moph a Mutalisk into a Guardian and a Hydralisk into a Lurker (Brood War) prove useful in the fact that they give the units life back. Also, the Zerg heal themselves at a slow, rapid rate. The Protoss just get thier shields back, and the Terran buildings and mechanical units can be repaired by an SCV. Perhaps one of the most useful things for the Zerg is the ability to burrow. This keeps units safe and unseen to anything but a detector. This is great for scouting. The Terran have nothing like it, and the Protoss can train Observers and Dark Templar (Brood War) which are permanently cloaked.
If you are going to be Zerg, there are certain things you will need to be really good. The best upgrades are Burrow, Hydraslisk Attack range, Zergling Speed, Overlord Speed and Sight Range. Those are all very useful and will help you a lot. The best units are Hydralisks, Queens, Defilers, and Mutalisks/Guarians. If you get those things you will be very sucsessful.
Because Zerg usually have many units, the worst things are the attacks that hurt many units. Some of these attacks are Psionic Storm, Plague, and Irradiation. Units that do splash damage are also very annoying, such as Siege tanks, Reavers, Infested Terrans, and the most deadly, Nukes. All Zerg units are Biological, which means that attacks that are most effective or can only be cast on Biological units are very useful on Zerg. Basically, anything that hurts many units at once is deadly to Zerg.
Zerglings are one of the best units in the game, even after you have used thier rushing abilities. In the early game, tell your first 6 zerglings to scout for the enemy, then meet back at one common point. You can do this by telling them to move in one place, then hold shift down and tell them to move to another place. Once you have found your enemy, you should have many more Zerglings. Attack that enemy while you get more Zerglings. your first wave of Zerglings should be about 12-18, but your second should wait for 24. If this doesn't kill your enemy, start climbing the tech tree.
Zerglings make great walls! When you are fighting Zealots and Firebats, Zerglings are useful to protect your Hydralisks. The Zerglings go up close to your enemy, forcing them to either go around your wall of Zerglings to your Hydralisks, or take out the Zerglings first. Either way, the Zealots/Firebats have most likely lost thier lives to inflict very little damage.
Burrowing is a great thing for the Zerglings. Because they cost so little, it isn't a bad idea at all to use these to warn you of enemy attacks and when your enemy is about to expand. Set burrowed Zerglings at the exit of any enemy base so that you can see thier troop movement, at any mineral field, and around your base as well. Also put these anywhere else you know that an enemy has troop movement. Another great place to put these is on tiny islands that are barely big enough for a Zergling, where no one will ever look for a burrowed Zergling. Burrowed Zerglings allow you to monitor your enemies well and be forewarned of an attack.
In midgame, Zerglings are still useful for purposes other than monitors. Keep a small mob of these that is about 12-24 to attack. If you attack with them, your enemy will probably go very defensive and wonder why you are wasting your money. When you are ultra defensive, your actions are more controlled by other players because you are always reacting to thier attacks. So while you are climbing the tech tree, keep attacking with Zerglings.
Here is an excellent attack that is rather hard to pull off, but can hurt any enemy critically. You need 12 Zerglings and 12 Hydralisks. Tell your Hydralisks to head strait for your enemies "Peon line" and kill all their "Peons". Then burrow your Zerglings while your Hydralisks kill thier Peons. Wait until your enemy has used his money to replace his all his Peons, then unburrow your Zerglings. Concentrate on killing all the Peons again. Now all that remains of your enemy is a way behind economy, some low tech units, and a puny defense that has 'attack me' written all over it.
Hydralisks are very good for about anything. Most air attacks are weak at ground units, so these make excelent defense for air. Later in the game, you will want many of these and less Zerglings.
Here is a cool trick to pull at the biggining of a game before Siege tanks arive. This also works with Zealots. Get 12 Hydralisks and label them group 1 with ctrl + 1. Attack your enemy until the first Hydralisk turns red. Then Burrow them. While you attacked you should have been getting more Hydralisks, so with the next 12 label them group 2. Attack until the first Hydra turns red and Burrow them all. Do the same for 2 more groups. Then press these buttons in the same order: 1, u, 2, u, 3, u, 4, u This should unburrow them all, and just watch!
Take many Mutalsks and many Hydralisks. Attack with both of them from one area. Make a hole for the Hydralisks to come in without getting hurt. Take out all the towers with the Hydralisks, then bring in the Mutalisks. Aim first for any towers, then for the Peons.
If you find yourself with a Mutalisk that is very low on life, make him a Guardian! The Guardian will emerge from its Cacoon with full life.
A way to take out a tower with a Mutalisk is to get several Mutalisks, a Zergling, and a Defiler with Dark Swarm. Find a safe, high area that is above a turret if possible. Otherwise, get level with it. Move your Zergling somewhat close to it, and tell it to 'Hold Position'. Cast Dark Swarm over him and move your Mutalisks into position. Tell them to attack the Zergling. What should happen is that the Mutalisk attack should bounce off of the Zergling, which is protected by Dark Swarm, and hit the tower. This works against Protoss to, but be careful of Psi Storm.
Besides the highly effective kamakazes, these make good scouts. Set several around your base, patrolling. They make good scouts this way, and are less killable by Psi Storm. If you catch sight of a incoming transport, it is very vulnerable and probably has a lot of troops with it. Take about 6 of these and tell them all to attack it. This way, if there is backup air, enough get through to the transport to kill it! One great danger to this scouting settup are Terran Cloaked Wraiths. If you are playing against Terran, upgrade Overlord speed and have them patrol too.
These are good to have many of because they handle air very well when combined with Mutalisks. They are great for taking out Battleships, Carriers, Transports, Scouts and Guardians. They kill their prey for a tiny fraction of the cost that the enemy had to pay!
One of the biggest threats to a mob of Scourges is a Hallucinated ship. Only let a few of your guys attack any ship, and if you can, try to oversee that they attack real ships. Always get replacements for your Scourges when they die. Keep them patrolling and spread out so that they don't all attack one target.
Queens are my favorite "Mage" in the game. All of their spells are very useful. The first is parasite.
Parasiting just about anything is good. But to be most effective there are specific things to aim for. Aim first off for the Critters. If you can get all the critters parasited, which isn't that hard, you can see half the level! But remember that your enemies can also parasite Critters. Kill any Critters that are close enough to any of your troop movement to expose you. If you see any Critters that are walking in a strait line and don't look like they are wandering aimlessly, it might be a Hallucinated Critter. Kill it! The next thing to aim for are the expensive units that are too valuable to throw away. Battleships and Carriers are good targets. Overlords and transports are great things to Parasite because you usually don't have a transport selected, and most people store there transports in their safest part of thier base. Parasiting Peons is also good because it can alert you to what they might be building and if they are expanding. If you get a drone parasited, try to make him your next building. That way the parasite goes away. One more great unit to cast this on is Wraiths. If they cloak and you have a parasite on them, they might as well save there energy!
The Queens have another awesome spell, Broodlings. An important key is to aim for the more expensive units, like Tanks, Dragoons, Ultralisks, ect. Another great thing to make Broodlings out of are Peons. Go to a peon line and get some broodlings. If you're lucky, their defenses are to far away to kill the Broodlings and they will have to move some troops. This gives you time to kill a couple of other Peons. If you keep a ton of Queens (I say about 8-12) you can Broodling an entire army in a short amount of time. If this happens to you, and your entire army starts becoming enemy Broodlings, start getting Scourges. When you hear 'we are under attack!', find where and attempt to get the Queen.
Ensnare is yet another excellent Queen spell. Use it on retreating guys. If you are attacked by scouts and overwhelm them so that they retreat, put ensnare on them right away and chase them with Scourges. 3 scourges annihilate a Scout. If you are beeing attacked with cloakers and you can tell where the cloakers are, cast Ensnare in that area to reveal any cloaked units.
The most dreaded thing a Queen can do is to Infest a Command Center. If you can get a Command Center infested, first high tale the Command Center to a secure location, then start Training Infested Terrans!
Ultralisks have very thick armor, and a lot of life. This makes them very useful for getting up close to things that are too defended for anything else to get to.
Many times, Terran players bunker up so much that you can't attack effectively without loosing a lot. If you can get an Ultralisk and a Defiler on the same level ground as the Terran defense, you can tear through it and not get hurt much at all. Take your Defilers and cast Dark Swarm so that it makes a long path that stretches from out of the range of the tanks all the way to the closest bunker. Walk your Ultralisk through the Dark Swarm and make sure that he doesn't get out of it. This will make him not get hit by any ranged unit. You will have to watch for FireBats though. The Dark Swarm will last long enough for you to take out a lot. Also, if your Defilers had 250 energy before you started this, they should soon have enough to cast Dark Swarm again. Keep doing this as long as you can. When you can't do it any longer, attack! The defense that once was big is now very vulnerable.
Ultralisk drops are not used very often, but if you can risk it you should. If you unload them in the peon line, you can take out a lot, even with your enemy firing at you. However, it is very expensive to have 2 Ultras loaded into a transport, and very often it is too risky and almost sure to be a loss of a lot of money.
Ultralisks are great for attacking other melee units. That is, they are great for attacking other non-range units. So if your enemy has a consitency of attacking with Zealots, Zerglings or FireBats, you can fend them off very well with a few Ulralisks and some Hydras. Zerglings and Ultras make great walls, protecting units such as Hyrdalisks very well. In joint what they will happen is that the Ultras will take all the hits, the Zerglings will be free to attack, and the Hydras will be unreachable to most things.
Dark Swarm can make 3 Hydralisks do as much damage as 30 Hydralisks. Position any units in a mob and cast this spell over them. They will be invulnerable to all range units but Scarabs. This is perfect for taking out Terran defenses (thanks Pestilence111 for the attack idea!), so move your units into a mob, cast the spell, and attack! For best results, use Zerglings and Ultralisks, because Hydralisks won't be able to attack anything that wanders inside the Dark Swarm. If you can manage it, lay Dark Swarm all over the place in an enemie's base so that there is one big blanket of it, and let loose a bunch of Zerglings. You will have a hard time seeing what is going on, but when the Swarm clears, you will have killed everything in the cloud without loosing a single unit (unless the enemy has no-range fighters, Zealots and Firebats).
Plague is good against anything. It drains life until it has either drained 300 or gotten down to 1-3 life. This is a safe way to take out the Terran defense! Bunkers will blow because the buildings will start loosing life when they are red. Tanks will die with one hit, and turrets too will blow. This is also great on any Protoss unit because Protoss are incapable of getting their life back, so all you have to worry about are shields.
I do not suggest consume unless you need to use more than 250 energy at once. Consume nothing but Zerglings! Anything else is a waste. Use what you can, then consume your Zerglngs, then use your next spell.
Guardians have the second longest range in the game, only second to the Siege Tank. They outrange anything that can attack them, including turrets. So attacking Turrets, Spore Colonies, and Photon Cannons is a very good job for Guardians. These are also one of the most mennacing things to the Terran Defense. The only drawback to them is that they are completely defenseless to air! They have potential to have very high armor, they have a very long range, and they have a very good attack. But they are very easily killed by air units. So keep them defended!
A good counter to these units is Goliaths. They can take a pretty large beating from Guardians, and can deal some big damage to them as well. For this reason, you should use Hydras to help take care of Goliaths and Dragoons. The Hydras will still be out of range of most units, and will help attack anything that gets close enough to attack a Guardian.
When you defend with these, or you are just storing them up, you should keep them from being in the front of your army. Store them behind Hydras and Colonies.
Infesed Terrans are kamakazes, and are not at all to be left as defense! When you aren't using them, keep them burrowed. That way they won't explode on a simple Marine and kill half your army. The only way you should keep them as defense is to kill any stragglers of an attack. They are great for killing the Reavers that come after the initial attack, and great for taking out sever Siege tanks since they usually station themselves in mobs.
When you attack, use these to take out any Terran defensive unit, any turrets/colonies/photon cannons and best of all... Farms! You can keep your enemies running off of low supplies by destroying farms! Most people put their farms in on area, so try to unload an I.T. in the center of 4 supply depots, and BOOM! Your enemy now is short by 32 supplies!
If you can't get your I.T.s far enough in to attack a bunker or Siege Tank, you can use Dark Swarm to help you achive your goal. If you do that, you also have more time to possition the Infested Terran just right before he blows up.
Infested Terran Drops are very effective, if you can get through turrets and stuff. Pack an Overlord full with 4 of them, and when you get to the spot you want to unload them, click on the portrait, so that it unloads only the one you clicked on. If you unload them all, one will blow up all of them, and you will have wasted them. If you get the the peon line, aim for the Command Center. One blow can take a third of its life, and kill many Peons. 2 blows will kill several peons, and damage the Command Center enough so that you can Infest it. Now they have much fewer Peons, and lack a Command Center at one of their bases.
Drones are the workers for the Zerg race. They are very useless when it comes to attacking with them, but they are useful around the base.
If you are attacked and your peons become defenseless, you should Burrow them. If you have an expansion, you will be ok, and you will have some extra Drones. But if you haven't yet expanded, once you loose all your builings your dead. If this is the case, use them to attack or run them away and try to start again.
If a drone becomes parasited, try to make him your next builing. The parasite will go away!
For Terran and Protoss races, you often don't get very many Peons. But if you are Zerg, you will need a lot if you plan to make a lot of buildings. Therefor, be sure to train a few extras.
A very important rule to always follow is to never use your last peon on a building. You may trap yourself if you do!
Overlords are one of the most important units for the Zerg. They serve many purposes. They supply control, they are the most abundant detectors, they make good scouts when upgraded, and they serve as transports.
At the beginning of the game, you shouldn't need very many Overlords. But later on in the game, they are more in demand. So keep several more than is demanded by control needs. Place them wherever you are vulnerable to cloaked units, or use them as scouts anywhere they give you an advantage. But be sure you keep enough so that if you loose several suddenly you wont be too setback.