For you unwashed heathens, the game is set in the Warhammer 40k universe where you control a squad or two of elite Terminators who run around a huge space derelict blasting the shit out of aliens which infest it, more specifically, Tyranid Genestealers.
Its a turn based tactical game, and your marines gain experience and are awarded better gear as they level up.
So, in the name of the Emperor, go kill some 'Stealers!

Oh, and a word of advice. Keep your marines away from close combat or chances are you'll probably see a messy stain on the floor.