Defuse The Bomb is the only game type where the team you are on affects gameplay in anything other than visual skins.
The Army (A grey circle with a black winged unicorn) trys to defuse a bomb on the map before it detonates. The Resistance (A red circle with a gas mask and two AK-47s in a "skull and crossbones" style position) try to stop them.
My suggestion is to use shotguns and movement speed if you are Resistance, because if you play defensively, the Army is going to win. Wait for enemies to run past you, then go after them and shoot them in the back with a high damage weapon (preferably the M4 shotgun, for its one hit kill ability and lightness.) you can also knife them if you want to go silent. You can camp if you are the only one doing it, and it is in a concealed position. Wait to see if anyone comes in after the first enemy before you shoot. if a player dies walking through a door, their runner-up will throw a grenade in.