Zerg Unit Guide
Hatchery Units Lair Units Hive Units
Drone
Drones are in the middle of the pack of worker units. They aren't as buff as SCV's or as fast as probes, and they must sacrifice themselves to morph into buildings. The upside is that with mutliple hatcheries, expansions can be manned in no time, and peon raids become much less devastating. Four hatches of 3-4 larvae each will automatically reproduce 12-16 drones in the time it takes to make one SCV/probe given enough minerals.
Overlord
The coolest of any supply or transport units. They're cheap, durable, fairly quick (with upgrade that is, but HORRIBLY slow without), and detectors. That makes cloaked units virtually useless against Zerg, because of all the detection. Great for corny early scouting, and convenient for massive drops because they are needed for supply anyway. Far and away the best non-combat unit in the game.
Zerglings are by far the weakest of the initial units, although in small numbers they can overwhelm Marines. The thing that makes Zerglings useful is the fact that they are cost effective, because three zerglings can beat one zealot or three marines. In higher numbers, zerglings can surround zealots and cost effectively smoke them unless zealots have a crucial attack upgrade descrepancy. Against marines however, after a critical mass of about 12 marines zerglings are not at all cost effective, because they are often shot down before they reach the group of marines. With upgrades, they can cost effectively beat hydralisks in open areas in Zerg vs. Zerg matches. They are the perfectly compliment to any other zerg assault unit, because they run in and clean up very well.
Crackling
A 3/3, fully upgraded Zergling is called a "crackling," because they are used en masse to crack late game defenses mainly against Protoss, but also with limited effectiveness against Zerg. Against a Protoss without Archons/Reavers or a Zerg without Lurkers, masses of cracklings can be overwhelming because they can destroy any stationary defenses very quickly and surround and massacre your defending units. Even High Templar have trouble with these because they can be sent in such numbers, they run out of psi storms. Zerglings are truly a valuable unit from early to late game.
Hydralisk
Second only to the marine as the best overall unit in the game. They are ranged, hit both land and air, and have an awesome cooldown speed. Their cheapness makes them the most massable unit in StarCraft, and plain old hydra masses are still quite effective. The reason they aren't any better than the marine is because although they have double the hit points, they still pop in one scarab or psi storm, so it really doesn't matter. Stim makes the marine better than the hydra overall, but hydras are definitely a worthy runner-up.
Amazingly effective infantry splatterers against all three races. Very devastating to any Protoss or Terran because of their burrow attack, which requires detection to defend against. They quickly massacre a peon line or expansion, and can unburrow and run away quite well. Unfortunately, they are the basis of many a gay anti-Terran strategy, a la Dark Templar.
Mutalisk
The mutalisk may be the most ill-designed unit in the game. They are too powerful in early game and too weak in late game. In original SC, they were THE topic of imbalance controversy. Blizzard actually had the cannon changed to normal damage and two new units in BW created specifically to deal with them. They are the best early air-to-ground of any race, and muta rushes are disastrous to anyone unprepared because of their speed and good ground attack. ATA, Mutas get slaughtered by Corsairs, Valkyries, and Scouts. Because Wraiths are so crappy, they fair well against them, but I wouldn't recommend many late game muta raids without Devourer and Scourge support.
Scourge
Crappy units. They are good early anti-Corsair and Carrier, but beyond that, pretty useless. En masse, Corsairs deal with Scourge before they can even touch Carriers, and upgraded BC's kill them before they can even do any damage. Muta+Dev is Zergs best ATA option.
Queen
The cost of Broodling ultimately makes Queens not too worthy of creating. 150 energy comes too late for it to be effective against tank pushes, and there's usually so many HT it's not worth it. Better to just save the cash and tech to Guards. Parasite and ensnare are pretty worthless against Terran because of medics, although they can be useful against Protoss Corsair packs. Queens are usually a case of too little, too late.
The absolute earliest and most cost effective ATG siege unit in the game. For 150/200/2 you can get a guardian, and although they can't hit air, their range and power makes them devastating for ground defenses. As compared with 550/250/8 (w/8 interceptors) for a Carrier and 400/300/8 for a BC, for cost, guards can't be beat.
Devourer
Awwwwwesome. Devs are the coolest ATA unit in the game. They hit the spot against Corsairs because their spores slow cooldown, and against capital ships by raping their armor. They're the perfect compliment to Mutalisks or Hydralisks, and in 2v2 situations, to your partner's Valks and Corsairs. A well microed Dev+Muta+Guard attack can often be a Finger of God of sorts against Protoss. Yeah, give those Carrier whores a taste of their own medicine!
Ultralisk
The forgotten Zerg unit. Never has been made, never will be made, and probably doesn't deserve to be made. In BW, two upgrades, speed and armor, were added specifically to help balance these poor beasts, but it still isn't enough. Cracklings do the whole late-game melee much more cost and supply effectively.
Defiler
The second coolest mage unit in the game, but are built less than friggin' Valkyries. Plague is fucking devastating and cheap in conjunction with consume. Ever had a group of BC's or Carriers plagued? It really sucks bad, especially for the irrepairable toss. Even buildings fall prey to plague. Dark Swarm+cracklings is instant death for most Protoss and Terran players, and consume and some zerglings gives you limitless energy. I guess some units just aren't meant to be built, but there's no reason that this should be one of them.