An automatic rifle is a rifle which is only capable of fully automatic fire. These are not usually select-fire weapons, and usually fire from an open bolt.
Often mistakenly defined as a "machine gun".
Machine guns are crew served weapons, meaning it generally takes 2 or more people to operate the weapon system.
An automatic rifle is issued to and used by an individual infantryman, usually in the place of the S.A.W. (Squad Automatic Weapon).