XMP is a class-based Team game where 2 teams fight to control "artifacts". Each team starts with 2 that are kept at their base, the goal is control all 4 of them. (Think CTF with 4 flags). Whats interesting about XMP (and fun) is the classes are all balanced to compliment each other in battle. Scattered across each map there are Deploy Points and Energy Sources. Deploy points obviously are a respawn point for when you die. Deploys outside the base are generally neutral and the teams fight to control them. The Energy Sources provide the team with energy. The more energy you have, the more cool things you can use, such as stationary deployable turrets, land mines, lazer trip wires, massive mounted turrets, vehicles, etc.
Hope that helps.