Civilian version of the M-16 assault rifle. Designed by Eugene Stoner and L. James Sullivan, based on Stoner's AR-10 design. Created as a prototype for the US Military, which was tested and small changes made, which resulted in the M-16. The Ar-15, Ar-10 and M-16 are all gas-driven, direct impingement systems with a light weight, high amount of modularity and a fairly high rate of fire.
Originally produced by Armalite, then Colt, now FN Herstal. But now has become a generic trademark for a rifle of this design by any manufacturer. Any non-military issue rifle of this design is properly labelled AR-15, not M-16 or M-4 which denote specific models issued by the US Military.
A common misconception is that the AR on the front of AR-15 stands for "assault rifle", however this is completely untrue. The "AR" part in fact stands for "Armalite Rifle," after the company that designed it.
But the shape of the rifle is so iconic the name stuck. Even the AR-15's conversion into pistol calibers is often still called an "assault rifle".
For artistic purposes the AR-15 is indistinguishable from the M16 or M4 so both tags should be used.