Starcraft II was broken into three editions in order to get the game out quicker, with Legacy of the Void hitting shelves on November 10th. This edition of the game finishes the story that has been running since the release of Starcraft 2 and is suspected to finally finish the story, unless there is DLC beyond this. You'll have a few ways to buy it. There may be a box copy hitting stores, and you'll be able to buy it electronically. That's about all there is for news in regards to the release date for SC2.
The expansion is $39.99 and as far as I know, it works both standalone and as an expansion on previous games. You get the base units, plus the expansion units, plus this expansion's story, however, you'll be far out of place without playing the other two games. I will say that StarCraft II single player is pretty boss and quite enjoyable, odds are we'll have some awesome guides to here at Ten Ton Hammer too if you need help getting started with it.
The expansion features a ton of new single player options, a prologue, and then two new units for each race, including Lurkers and Ravagers, with the Lurker coming from Starcraft 1. New co-op game modes, and well tons of things. If you prepurchase now, you'll get access to the beta, along with a head start on the three prologue missions. Otherwise, the game will launch November 10th, roughly a month from now.
To read the latest guides, news, and features you can visit our StarCraft II: Heart of the Swarm Game Page.