RPGs are frequently measured in how many hours it would take someone to complete the whole game, but with Mass Effect, perhaps light-years would be a more effective yard-stick. Our private viewing of this game revealed an adventure so huge, it strains the capacity of your brain to attempt to encompass what it's all about... but we're going to try anyway (just for you).
First off, you play as Commander Shepard, a sort of space federal agent in the employ of a peace-keeping organization called the Specters. You can use the out-of-the-box character model that Bioware provides (he looks a little like Cillian Murphy, the star of 28 Days Later) or you can create your own character. You can even make Commander Shepard a girl if you want and everyone else in the game will refer to you by your title, so as not to introduce confusion. Everyone else in the game also has a voice and those that adventure with you (you can have up to two squad-mates) will frequently comment on your actions.