SAW - Squad Automatic Weapon

A squad automatic weapon (SAW, also known as section automatic weapon or light support weapon) is a weapon used to give infantry squads or sections a portable source of automatic firepower. Weapons used in this role are typically either light machine guns or selective-fire rifles, usually fitted with a bipod and heavier barrel. SAWs usually fire the same cartridge as the assault rifles or battle rifles carried by other members of the unit. This reduces logistical  requirements by making it necessary to supply only one type of ammunition to a unit. SAWs are light enough to be operated by one man, as opposed to heavy machine guns such as the Browning M2, which fire more powerful cartridges but require a crew to operate at full effectiveness.

Many SAWs (such as the RPK and L86) are modified assault rifles or battle rifles that may have increased ammunition capacity and heavier barrels to withstand continued fire and will almost always have a bipod. In the case of some assault rifles, such as the H&K G36 or Steyr AUG, the SAW is simply the standard rifle with a few parts replaced. The most common SAWs in use today are derived from two basic patterns: RPK or FN Minimi. One of the first weapons designed for this role was the M1918 Browning Automatic Rifle, which, though having a limited magazine capacity, was still more than that of the typical infantry rifle, and it gave the infantry a base of fire weapon that was more suited to maneuver warfare than the bulkier machine guns of the period, such as the M1919 Browning machine gun.

source : wikipedia