Starcraft II: Heart of the Swarm is the first expansion pack for Starcraft II and is considered part two of a three part trilogy. Heart of the Swarm will include new units and will focus on the Zerg.
This expansion pack will offer around twenty new missions and will hopefully be released sometime in early 2012 for the PC and MAC.
Here is the teaser E3 trailer for Starcraft: Heart of the Swarm:
And here is a teaser for the gameplay: