A Squad Automatic Weapon (SAW, automatic rifle) is an assault rifle or, in some cases, light machine gun, that is equipped for full auto functionality. Squad automatic weapons are either adaptations of an assault rifle, generally with a larger magazine, a longer barrel, reinforced receivers, etc, or a weapon design in its own right.
In a military context, the squad automatic weapon is generally an assault rifle that resembles the service rifle, but with the selector switched to AUTO, as opposed to SEMI. If the squad automatic weapon is a light machine gun, as it often is, the weapon will only have full auto functionality.
The SAW is used for suppressive fire, and to augment the firepower of the squad; the automatic rifleman carries much more ammunition than other squadmates.