Zerg
The zerg are a species of biologically advanced aliens. They are the third of the three dominant and playable races in the game. The zerg are considered the least technologically advanced of the three races. The zerg are a species aiming for genetic perfection, and assimilate advanced species across the galaxy, incorporating beneficial genetic code into their own.

Drones, like the probe and SCV, are the basic workers of the Zerg. However, drones are kind of different from the other two starting units. In order to build structures, drones need to sacrifice themselves to commence the construction of the building. They also can burrow, which makes them invulnerable to enemy units unless they bring along a detector.

Larvae are small units produced at a hatchery, lair, and hive. They basically are the first stage of nearly all zerg units, though there are some units which cannot be produced from the larvae. such as infested terrans, and broodlings. Larvae cannot attack, and have very few hit points, but they take forever to kill, and don't ask me
why.
Overlord
The overlord is the only unit in the game that acts as a supply provider and detector at the same time. They can fly and have no attack. they are slow, but often act as scouts due to their far sight range. They are quite durable and take a long time to kill without a significant force. They also can be used for zerg defenses due to their ability to detect cloaked units.

Zergling
The zergling is the basic melee unit of the zerg. they are cheap and easy to produce, but are weak compared to the other two basic units. Zerglings are useful in early-game rushes and are very fast, allowing them to overun other enemy units in swarms. However, they are weak individually and can be decimated easily by siege tank and bunker defenses.

Hydralisks are one of the most important units in the game, due to their ability to attack both air and ground units. They are effective in groups, but there are many counters to hydralisk groups. Siege tank and bunker defenses, as well as reavers, psionic storm are able to easily kill off entire swarms of hydralisks. Still, hydralisks are cheap enough to mass produce and can play key roles in the game.

The defiler is a unit appearing in the late-game process. The defiler has no direct attack but can cast spells that can help you in battles. Spells of the defiler include Dark Swarm, which prevents friendly and enemy units from taking damage from ranged attacks. The defiler also can burrow like other zerg units, and they can't be killed without a detector.
Lurker

Lurkers may be powerful, but they are useless against aerial units, so if you want to dispose a group of lurkers, just bring a science vessel (or observer if you are a playing Protoss), along with a few other aerial units that can attack, and then dispose of the noticeable hol
es in the ground. You will see a beautiful explosion of blood as the lurkers blow up, a spectacle I do enjoy.
Ultralisk

Ultralisks take a long time to produce, and are somewhat ineffective individually, especially in the late-game, when more advanced units begin to emerge. Therefore, it is best to use Ultralisks in groups to cause maximum havoc on enemies.

Infested Terran
Infested terrans are mutated versions of regular marines. They do not have a regular attack, but can explode, dealing 500 attack to all ground units around the area. This sort of damage means that any unit hit with the explosion will certainly die. Despite the usefulness of the infested terran, they are hard to produce due to the complicated process required to make an infested command center, the place of their production.
Mutalisk
The mutalisk is a zerg aerial unit that is mainly used for hit-and-run tactics, such as attacking stray units and buildings, then flying away before reinforcements can come. However, mutalisks are weak in head-on aerial battles, and are easily taken out by other flying units, such as wraiths, scouts, corsairs, etc. Mutalisks are also fairly easily defeated by goliaths.

The scourge is basically a kamikaze unit, hurling itself into another aerial target and exploding. However, this suicidal attack is not in vain. Dealing 110 damage to aerial units, a singe scourge can severely cripple any light air unit at full health, and a group of scourges can take out a carrier or
battlecruiser. The scourge is only available to use in the original Starcraft, and was not included in Brood War.
Despite being able to deal a devastating amount of damage to aerial units, the scourge has a very low number of hitpoints itself, which allows a group of units with considerable attack to decimate a group scourges if there is enough distance between them. Groups battlecruisers are dangerous to scourges, as a battlecruiser can take out a scourge with a single shot. Therefore, I'd advise you to only use scourges against individual units.

Queen
The queen is a zerg aerial unit which has no normal attack, but is rather a spellcaster unit, which means that they have a few abilities to help change the tide of the battle (Or at least harass the enemy).
The queen is meant to be used as a hit-and-run unit, just like the mutalisk. The queen can use four abilities, two of them starting abilities:
Infest Command Center (Starting ability): This ability allows the player to convert a badly damaged terran command center to an infested command center. When the building is converted, then it is restored to full heath. Infested command centers can be used to produce infested terrans, a suicidal unit (See above).
Parasite (Starting ability): The queen attaches a parasite to an enemy unit. The controller of the parasite can see everything the infected unit sees, which can possibly give out important enemy locations to the user, putting the parasite controller at an advantage.
Ensnare (Evolved at Queen's Nest): All units in a 4x4 area have their speed reduced by half and their attack reduced by varying amounts. Besides this, al cloaked units in the area are revealed for the duration of the spell.
Spawn Broodling (Evolved at Queen's Nest): Two broodlings are targeted at an enemy ground unit. The queen fires the broodlings at the target and it is destroyed instantly. Out emerge two broodlings, which unless killed, will last for about 180 seconds. This ability is useful in destroying valuable units such as siege tanks and ultralisks.
Guardian

The guardian is an evolution of the mutalisk. To be formed, a mutalisk needs to morph into a guardian, this ability available after the construction of a greater spire. The guardian is devastating against light ground units, as their armor is strong enough to withstand the attack of even multiple light units attacking at once. The guardian has a ground attack range that outreaches nearly all ground-to-air units. However, they have no aerial attack and are slow, making guardians useless against air units.

The devourer is another evolution of the mutalisk. To produce them you need to evolve them from mutalisks. Devourers attack with corrosive acid, and for every attack, the target is sprayed with spores. Any unit that has spore attached will have its attack speed slowed by 1/8 per spore, and take 1 point of additional damage from every attack for every spore attached. Cloaked units will also be revealed if they are covered with spores. Given time, spores will disappear.