Specifically, it transforms you from Galen Marek, the Sith Apprentice known as Starkiller, into Deak Starkiller, elder brother of Luke Starkiller, from a very early draft of the A New Hope script. As part of the deal, you get an option to use Deak's signature yellow lightsaber.
The character design is based on a mildly famous production painting by Ralph McQuarrie, the grand old man of Star Wars concept art, which depicts Deak dueling Darth Vader aboard the Rebel Blockade Runner. Yes, Deak was the person Luke had to rescue in this draft, not Leia--a good thing it was changed, I'd say.
Preordering the STFU2 Collector's Edition can be done through that infamous spawn of Satan, GameStop. No, I won't link to them. But if you want this edition, you probably should visit them and preorder, due to its limited numbers. Again, this special version is available only for Xbox 360 and PS3.
Source: Twitter