Achieve one of the following goals on Xbox Live to unlock the corresponding medal:

Biohazard: Retrieve the canister 100 times
Communications Specialist: 250 Total Conquest captures
Interceptor: Get 100 flag carrier kills
Mercenary: Get 100 kills
Perfect Attendance: Play seven consecutive days
Purple Heart: Get killed 100 times
Sapper: Plant 500 explosives (no kills required)
Veteran: Visit all modes on all levels for each mode (Total Conquest, Team Sharpshooter, Team Survival, and Retrieval)