This is an assault rifle. It was designed at the Armalite division of the Fairchild Engine and Airplane Corporation, specifically by Eugene M. Stoner. Its design was sponsored by the US military as a candidate to become the next generation main battle rifle for US troops in the late 1950's.
The US military was adjusting its opinions on what features were important for a main battle rifle. Past main battle rifles were good for very few well aimed, long range shots (600-1000 meters). Their magazines held fewer rounds, they did not have high rates of fire, they were heavy, and their ammunition was also heavy (heavier bullets are more accurate for long range shooting than lighter bullets). The future main battle rifle would focus more on a higher rate of fire, higher capacity magazines, and it needed to be better suited for short range shots (100 meters). The AR-15 was designed to meet these new goals.
The AR-15 design was adopted by the US military as the M-16. Its specifications were as follows:
I have one of these, an actual Armalite brand AR-15. Due to assault rifle bans in California, it's impossible for a resident of my state to acquire one now. And I had to go to the local police department and have my thumbprint taken and put on file. Which, as many fear, is the first step towards confiscation of such rifles. That's something I am afraid of, because I am glad I have my rifle. It's very accurate, easy to operate and maintain, and is better than one would think for long range target shooting with the addition of an accurized heavy barrel.