Warhammer 40k: Space Marine may be centered around the Ultramarines, but the multiplayer is a totally different story. Space Marine 16-man multiplayer pays homage to the franchise's tabletop roots by implementing an extensive customization system that lets players swap color schemes and armor components. You can choose from existing chapter colors or create a totally new Marine for your killing pleasure.
Gameplay involves three classes -- Assault, Tactical, and Devastator Marines -- and a number of game modes across a variety of maps. For anyone unfamiliar with 40k, Tactical Marines are the balanced units, well-versed in every aspect of combat, while Assault and Devastator units serve more specialized roles. Assault Marines are the guys with jet packs, launching themselves into the thick of battle and armed with trademark Chainswords. Devastators tend to stay further back, where they unleash death by way of Heavy Bolters and Plasma Cannons. Every class serves a purpose, so use them accordingly.
The maps themselves are designed to suit class-based combat, like having a vertical design to accommodate Assault Marines launching through the air and underground routes to serve as alternate paths. Three maps have been shown to the public thus far, and Relic no doubt has more under wraps until Space Marine comes out on September 6.
Be sure to check out the new multiplayer screenshots as well. Obviously, the armor colors you see won't be set, but they do a good job of setting each class apart for now.