Blatantly taken from Wikipedia:
The AR-15 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation. It is manufactured with the extensive use of aluminum alloys and synthetic materials.
The AR-15 was first built by ArmaLite as a selective fire assault rifle for the United States armed forces. Because of financial problems, ArmaLite sold the AR-15 design to Colt. The select-fire AR-15 entered the US military system as the M16 rifle. Colt then marketed the Colt AR-15 as a semi-automatic version of the M16 rifle for civilian sales in 1963. The name “AR-15” is a Colt registered trademark, which refers only to the semi-automatic rifle.