The first appearance of submachine guns dates back to 1915, but the first mass use of them was seen during WWII. Most armies from both fighting sides had their troops armed (partially, of course) with many kinds of submachineguns. After WWII, SMGs became standard issue for members of fighting forces and support vehicles, for assault teams, and even for special police forces.
SMGs usually eat standard or improved auto-pistol ammo, and have the size and weight comparable to the largest of handguns. Providing ability for greater firepower with auto-fire, SMGs are good offensive and defensive weapons while range is generally lesser than 200 meters.