An assault rifle is not a general term for "military-style" weapons, but is a weapon built to meet a specific set of assumptions relevant to ground combat. Its most distinctive characteristic is that it fires a cartridge intermediate in power between the pistol rounds fired by submachine guns, and a high-power rifle cartridge. The heavier-than-pistol round gives more stopping power and range, but the lighter-than-full-rifle recognizes that long-range fire has largely become the function of crew-served weapons such as machine guns.
While the original assault rifles, like submachine guns, were capable of full-automatic fire, most current assault rifles are "selective fire", meaning that they can be set to fire short bursts, but not a continuous stream. Full automatic weapons are hard to control and require more ammunition to be carried. Still, assault rifles have 20 or 30 round magazines rather than the 5-8 round magazines of traditional infantry rifles; they are intended to generate enough fire to suppress an enemy without the need for individually aimed shots.
Perhaps the best-known examples are the U.S. M-16 and the Soviet AKS-74, which fire, respectively, 5.56mm and 5.45mm ammunition rather than the 7.62mm rifle rounds of their predecessors, the M-14 and AK-47. Some legal definitions of assault rifle consider them to be any weapon with a high-capacity magazine, but this is not the military usage. In like manner, the Soviet PPSh41 and derivative "burp guns" are not assault rifles because they fire a pistol cartridge.