IGN found out that during an Activision Blizzard third quarter 2012 results call, Heart of the Swarm was revealed to be scheduled for the first half of 2013.
They also stated that StarCraft II: Wings of Liberty has sold 6 million units. They also mentioned that BlizzCon 2013 is confirmed.
The multplayer part of Heart of the Swarm has been in closed beta testing for some time, featuring new units and mechanics for the Protoss, Zerg and Terran armies. You'll see more about Kerrigan and the Zerg in a single-player story as well.
StarCraft II is a very popular real time strategy game that yet still remains one of the few non free to play options, though Blizzard admits they're looking into that option.
If your memory needs some refreshing, check out StarCraft II: Heart of the Swarm's preview trailer below.