Drone:
The Zerg's worker. These are the economy of the Zerg and able to morph into the various structures of the Zerg. Not much to say except that they're very fragile, yet make better scouts than overlords.
Overlord:
Your supply limit is controlled by these flying balloons. Early in the game they're used for scouting, yet very slow at that. However as the game progresses, these units become very valuable, as detectors, transports, and supply. Losing overlords will slow down a Zerg player, especially since they're very vulnerable.
Zerglings:
The best early scouts in the game. Given the fact you get two from one egg, you can easily outnumber early forces. They do the most damage for their cost, yet they're also very weak. Their uses are very limited in the beginning. They're only good for rushes and scouting. It's only later in the game, once you adrenal glands, are zerglings useful units again.
Hydralisk:
These monsters are one of the most cost efficient units in the game. Given that they're useful against any unit, and can be created in huge numbers, they become the Zerg's main offense. They are far stronger than a marine, being able to take hits from a siege tank, and can deal out more damage to air units and defensive structures. However they do lose to stim packed marines, especially with medics.
Lurker:
The lurker is the Zerg's best defensive weapon early in the game. They are able to slice through packs of marines and any other pack of ground units. Though they are very expensive, I wouldn't place my offence in these units.
Mutalisk:
Mutalisks are the Zerg's earliest air threat. In packs of 9 or more they become dangerous, and are very fast and cheap. It's not a good idea to attack with anything less than 9, since they don't deal a great amount of damage are are weak. They make excellent scouts.
Scourge:
Zerg's anti-air missile. These are mainly used for targetting dropships, and killing the cargo within. They are very cost efficient against everything, except overlords. Though it's not a bad idea to take out overlords, only if it'll buy you more time. They are very weak too, and don't do too well against lots of wraiths, scouts, mutalisks, and battle cruisers.
Queens:
Queens are one of the best units in the game. Though they're also one of the most expensive. All their spells are good for any part of the game. Broodling can take out any tank push, and ensnare can stop any marine rush. Also breaking into a Protoss's base is impossible without queens. Infested Terrans' is just something extra to play with, though they aren't very useful. They also possess one of the best scouting spells, parasite. Parasite's range is incredibly long, making it easy to target a unit. Also queens are fast flyers, making them good for scouting as well.
Guardians:
One of the most feared units a Zerg player has. There flying crabs can out range any defensive structure, and can deal enough damage to eliminate any ground force as well. But of course, they have no air attack, and are very slow. They are also very expensive, making them very valuable, they require a lot of gas to create and to protect.
Devourers:
A Zerg unit that no one really cares for. It's uses are to counter capital ships. Not too useful, except as and mainly only a support unit.
Defilers:
These spell casters are very powerful, since they have the ability to regain lost mana instantly by consuming. They possess one of the strongest spells in the game, dark swarm. This combined with zerglings will break any defense. Also plague is useful for taking out mutalisks, wraiths, and scouts.
Ultralisk:
This unit is very usefull in ground attack, but you should not use them without air support, heavy damage dealers especially with the 5th armour upgrade and quikness (B.W. only) ).
