Camouflage, or camo for short, is a pattern applied to a given firearm.
Camouflages are normally used by the military for concealment in vegetation or other environment where a weapon's normal color would stand out, but are available to hunters and the general civilian population as well. With the advent of black firearms, such as the Glock and the AR-15, camouflage has become especially valuable; black stands out in all environments except for complete darkness.
Most camouflage patterns are a woodland color (which uses shades of green and light brown), but other camouflage can be used (i.e. shades of white and very light gray for snowy environments, shades of light tan and light brown for deserts, shades of light gray, white, and gray for some mountain environments).
Camouflage can also be used on gear, such as vests, helmets, etc.