Blizzard has revealed some of the multiplayer units for StarCraft II and a video of them in action as well. The expansion will focus on the Zerg, but the other two races will get new units and abilities as well.
Zerg units will include the artillery unit the Swarm Host, the Viper which is a "battlefield manipulation unit" with two new abilities.
The ability Ultralisk can be used to burrow underground and pop back up for fast attacks, and Hydralisk is a speed boost that allow the zerg to be more mobile when close to the hive.
Changes to Protoss and Terran were also detailed. GamesRadar has the list through here.
There's also a new multiplayer video below which Blizzard released earlier.