An Unmanned Aerial Vehicle, or "UAV" for short, is an aircraft that is controlled via remote control. UAVs normally partake in numerous combat roles, although most UAVs are employed in tactical roles, such as reconnaissance.
Standard UAVs are normally used in vigilance roles, such as scouting, recon, and surveillance. These are not equipped with any ordnance, so they cannot attack.
UAVs are not used just for intelligence purposes; if the situation demands it, armed UAVs can be deployed. UCAVs (short for Unmanned Combat Aerial Vehicle) can be used for several battle roles, including surgical/precision strikes, bombing runs, and all-out air combat.