It follows the tidings of the secret apprentice of none other than Darth Vader (you play as Vader during the tutorial, if you're interested) and his struggle to find a place by Vader's side. The main idea is that "Starkiller" (as he is affectionately known by others) has to go around after the devastating Order 66 and mop up the last remaining Jedi that are still kicking around and causing a bit of trouble for Vader and the Emperor. Not to give too much away but there are few decisions you to make that ultimately swings which way the story goes and whether you will stay with the Dark Side of the Force but it's not completely your choice.
The game play is practical and an arsenal of Force powers are at your disposal as the game progresses with force pushes, grips on objects, lightning and chokes all making appearances in the game. There's a number of enemies too from the classic Storm troopers (Starkiller is meant to be secret so no-one's allowed to see him, even if they are on his side :S), Wookies and, of course, Jedi. There are also a few rudimentary puzzles that need "cracking" as it were, but they're not strenuous and happily fit the BBFC's 12 age rating due to their simplicity.
All in all, I've always had a blast playing this game even if there are a few things that can be boring such as the loading times, similar enemy types and it can get a bit too easy if you master basic combos. However, these factors shouldn't stop you from playing such a great new direction in the Star Wars franchise. If you haven't played it, pick up a copy... it's dirt cheap these days. If you have, then pick up Force Unleashed 2 which is basically more of the same... Which can only be good, right?