“We are the Swarm. We burned a path of annihilation across the stars. And then, we vanished. Four years ago we aded into the shadows. We thrived¡­ we evolved. And now the time has come for us to step back¡­into the light. Now, worlds will tremble once again. Empires will fall. Alliances will shatter. We are the Swarm, but we are becoming much, much more. For the final war has only just begun.”

Blizzard introduced New units (With some old units)

Previous Zerg Units

Overlord
The Overlord lost its detection ability, but apparently it gained the ability to attack. Apparently it can also create small creep areas.

No apparent changes so far for Zergling, Drone, Lurker, Hydralisk, Mutalisk.

Infested Terran
The Infested Terran is a Terran Marine that lost the suicide attack and gained ranged attack just like its Terran counterpart.

Queen
The Queen seems to be the Zerg version of the Mothership, as it gained various abilities and you can only have one at a time. One such ability is that it can create a tunnel to reach any hatchery on the map.

Ultralisk
The Ultralisk has splash damage and it can also burrow.

New Zerg Units

Roach

Fast melee unit with low HP but high regeneration.

Infester

The new Defiler, it can cast Dark Swarm and it can also infest Terran buildings such as Barracks and Command Center.
Corruptor An air-to-air unit, it can turn enemy units against each other. The attacked units are rendered immobile and the enemy units will no longer treat them as allies.

Swarm Guardian
An evolved Guardian with longer range, more attack damage and the ability to spawn Broodlings.

Overseer
An unit that evolves from the Overlord, it has longer vision range and detection.

Nydus Worm
The new version of the Nydus Canal, it is a unit that can create canals anywhere on the map.

For Screenshots & More Info visit Starcraft2.com