The larvae, essentially enlarged forms of the original Zerg insects, contain the DNA of all of the Zerg breeds. They are able to evolve into any of the Zerg broods. They are produced by Hatcheries, and in Starcraft II, by Queens as well.
Drones are the most essential unit in the Zerg swarm. They not only mine for Minerals and Vespene gas, but also transform into the "buildings" that compose a Zerg base. Like Terran SCVs and Protoss Probes, they have incredibly limited attack capabilities, making them vulnerable to almost any attack. On the plus side, they can burrow into the ground to escape attackers. The main downside to Drones, however, is that drones do not "build" hive structures, they transform into them; every time you create a building, you lose a Drone. The Zerg tutorial mission goes out of its way to warn you never to use your last drone to create a building.
The most numerous of the Zerg units, Zerglings aren't especially powerful, but they more than make up for their individual weakness with vast, overpowering numbers. Their genetic code is so simple that two hatch from each larva, yet they only take up one population space. They can also increase their speed once researched at at the Spawning Pool. The main weakness of the Zerglings is that because they only attack with their claws, they lack any defence whatsoever against aerial units, although they can burrow underground to escape such attackers.
Hydralisks are one of the most famous Zerg breeds, representing the species on the original Zerg variant cover art and the episode selection screen in StarCraft. Although they possess incredibly powerful claws, they are far more dangerous from their ability to shoot bone spikes at such velocities that they can penetrate Terran neo-steel armour. They can attack both air and ground units, making them a very important factor in the Zerg swarm. Their downside is that they aren't particularly fast, though their speed can be upgraded. Like most other ground units, Hydralisks can burrow to escape enemies or lay in wait for unsuspecting prey.
In the StarCraft: Brood War expansion pack, Hydralisks gained the ability to evolve into Lurkers. While they are completely defenceless on the ground, when burrowed, they are able to shoot super dense spikes through the soil that impale anyone or anything found near that area on the surface. As such, they're perfect for defence, and are often used as "mobile turrets" to push the front line of a battle forward. Though Lurkers did not initially reappear in StarCraft II's multiplayer mode, they made their competitive reappearence in the final expansion, StarCraft II: Legacy of the Void.
One of the most useful Zerg units, Overlords relay commands from the Cerebrates to the rest of the Zerg swarm; Zerg players rely on Control to expand their population, and must spawn more Overlords in order to build more units, in the same way that Terran players must build supply depots and Protoss players must build Pylons. How the Overlords communicate with the Zerg and how they survive the vacuum of space is unknown, but they do care for the larvae and transport other units within their bodies. They can also detect cloaked and burrowed units.
In StarCraft II, they now have the ability to create creep for producing buildings. In StarCraft II, Overlords are no longer detectors, but can evolve into Overseers, which are detectors, but do not have the ability to transport units from place to place or create creep. Instead, Overseers can create changelings, tiny units with a limited lifespan who transform into enemy-coloured Marines, Zealots, or Zerglings when they encounter an enemy unit, to serve as the perfect spy, as well as a contaminate ability that sprays slime on an enemy building and inhibits it from producing units or upgrades for 30 seconds.
The basic Zerg flying unit. Mutalisks are able to shoot a small parasite known as a Glaive Wurm out of its mouth at enemy targets that bounces from one unit to another while disintegrating explosively. It is able to stand up to the other races' flying vehicles quite well, and their relative cheapness means fielding a large 'air force' is easy. In Starcraft, Mutalisks are capable of evolving into two different specialized attack forms, the Guardian and the Devourer, but in Starcraft II, neither form makes an appearance.
A flying unit that can only attack ground units. It is an evolved form of the Mutalisk. Guardians are considerably slower than Mutalisks but have an incredibly long attack range and, as its name suggests, a small group of them can defend an area from ground units with ease, though if protected from aerial attacks, they can also serve as heavy bombers. Guardians were intended to return in Starcraft II, with the new name Swarm Guardians, and were present in several builds, but have since been removed entirely, their function now taken by the Brood Lord. As with the Lurkers, it has been suggested that Swarm Guardians will appear in the Heart of the Swarm campaign.
The other alternate form that a Mutalisk can evolve into, added in the Starcraft: Brood War expansion pack. Devourers are air units that can only attack other air units, spewing a highly corrosive acid that can melt through almost anything. Their acid also effects units around the one that it was targeting, literal "splash damage". Although the attack itself does not deal very effective damage, the acid slowly lowers the HP of the unit and, notably, slows down the speed at which that unit can attack. Like the Guardians, the Devourer form does not appear in Starcraft II, but like the Defilers, Devourer corpses can be found and collected to earn Zerg research.
Scourge are flying suicide bombers that ram other aerial units and violently explode. They, like Zerglings, can be hatched two at a time from a single larva. Because they are cheap to produce and incredibly strong (four or five can take down a mighty Terran battlecruiser), they make a very efficient counter to aerial units. Scourge do not appear in StarCraft II's multiplayer mode, but have been sighted in the Wings of Liberty campaign, so it is likely that they will be a campaign-exclusive unit in Heart of the Swarm.
The Queens from Starcraft and Starcraft II are completely different units.
In the original Starcraft, the Queen is a flying unit which has no attacks and relies on special abilities. It is able to Ensnare an area, reducing the speed of any unit caught in that area and revealing any hidden units. They are also able to attach a parasite to an enemy unit, allowing the Zerg player to see everything that unit sees. If the unit is a Detector, the player will even be able to see any invisible units near them.
They are also able to Infest a Terran command center, once damaged into the "red zone" of its health bar, which transfers control of it to the Zerg player and allows them to build Infested Terran units. Finally, they are able to infect a biological unit (or a vehicle driven by a biological unit, such as a Terran Siege Tank or Protoss Dragoon) so that it will instantly die, and two Broodlings will expel from the corpse.
In Starcraft II, the Queen is the Zerg equivalent to the Terran M.U.L.E. and the Protoss chronoboost: a support mechanism. The Queen can spawn additional larvae from the hatchery, increase the spread of the Creep, and heal friendly Zerg Units. Now modified to be a ground unit, the Queen moves moderately quick while on the Creep, and slows to a crawl while on normal terrain. A middling ground attack and strong anti-air capabilities is another departure from the role the Queen played in Starcraft.
Broodlings can only be created when an original Starcraft Queen infects an enemy unit. They have a limited life span although their existence means an enemy unit died to give birth to them. They are not powerful attackers, weaker in every way than even the Zerglings, but a handful of Broodlings can be a good distraction in a battle.
In Starcraft II, Queens no longer have the Spawn Broodlings ability and they are instead brought to the battlefield by a new unit called the Brood Lord, which generates them inside its body and hurls Broodlings at ground targets as its primary attack, instead of spawning them inside enemy units. Once the Broodlings have impacted their target, they bounce down to the ground and begin attacking things of their own volition, making the Brood Lords a dangerous threat.
Healthy Terrans are turned into expendable pawns if a Zerg Queen can infest a Command Center. Unlike Sarah Kerrigan and Samir Duran, psychic Terrans whose personalities remained more or less intact upon being infested, these Infested Terrans are mindless drones who live solely to obey the will of the Swarm. They are suicide bombers who run into buildings or ground units and explode in a massive release of caustic chemicals. They are capable of burrowing in order to set up ambushes. Because of the difficulty inherent in obtaining them, and the suicidal nature of the units, infested Terrans cause more damage per attack than any other unit in all of Starcraft.
In Starcraft II, instead of being suicide units, Infested Terrans use the rifles they were given in their previous lives as marines, and they are not created by Infested Command Centers, which no longer exist, but are instead inexplicably spawned by the Infestor unit, who can create pods from which Infested Terrans emerge. At this point, they have a limited life span, after which time they drop dead.
Defilers are considered the most sinister of the Zerg breeds. They produce various viruses and poisonous chemicals. They often have other Zerg escorting them so that they can devour them to produce these chemicals faster. Defilers have the ability to produce a plague that damages nearby units over time until their HP is 1, rendering them vulnerable to, well, everything. They also have the ability to emit a dark swarm cloud that allows units inside of it protection from ranged weaponry. Defilers do not appear in StarCraft II, but their corpses appear in one level in the Wings of Liberty campaign, where they can be 'harvested' by the player to earn Zerg research.
The Ultralisk is the most physically powerful unit in the Zerg swarm. They are massive walking tanks, tearing enemy ground units asunder with their massive blade-like tusks with ease. Their biggest weakness is that they have no ranged attack or any defence against air units, apart from retreating. It gains the ability to burrow in StarCraft II.
An extremely advanced Ultralisk known as Torrasque appears several times in Brood War. It is far more powerful and more armoured than a regular Ultralisk and can withstand dozens of Siege Tank volleys before going down, and even then can be reincarnated by the Swarm, in the manner of a slain Cerebrate. It first appears on Char as an obstacle for the UED in the Terran Campaign but appears later on Braxis as an optional unit for the Zerg Campaign.
In Starcraft II, an even bigger breed of Ultralisk, the Omegalisk, was seen in a mission representing a vision of the future.