Space Marine is a game set in the Warhammer 40K universe. It's...kind of a long story. You can find out a lot more on
wh40k.lexicanum.com
.
Here's the abridged version. In the very very far future, humanity has expanded into the stars, encountered fascinating alien races...and decided to kill them. After a period of expansion, the human empire collapsed, and was then reformed under the Emperor of Mankind. The Emperor's elite troops are the Space Marines, and they look like this:
The game is an over-the-shoulder shooter similar to GOW, but unlike GOW does not use a cover system. You actually can only regain health by killing enemies (using an "execution" move that is particularly brutal). Multiplayer pits you against the Space Marines' "heretical" brothers, the Chaos Marines (former Space Marines who basically started worshiping demons). It's 8v8 gameplay, either in team deathmatch or a control point mode with both sides on attack. You have three classes, but a lot of variation within those classes.
The gameplay is pretty fun, but the matchmaking is a bit dodgy currently. I'm hoping they'll fix that soon. They'll be releasing co-op gameplay some time in October, so that at least will be fun to play with folks. That should be similar to L4D but in over-the-shoulder vision. In that, you'll fight hordes of Orks. (Yes, Orks. Warhammer 40K is basically the sci-fi version of a fantasy game, although they've tried to move it further away from the fantasy roots in recent years. Orks are still badguys, though.)
Anyway, I'm digging the multiplayer on the whole, although it takes a bit of getting used to. When there's lag, it SUCKS, but when it's smooth, it's fun to play. My favorite is playing as an assault marine (where you have a jetpack and a chainsword). But there's also a heavy weapons class, and an all-rounder class. Give a yell if you pick up the game.