See also; SMG
It is generally used in close combat where accuracy is less important than rate of fire. The usage of pistol ammunitions means lower recoil than conventional rifles, thus enabling it to be fired at high rate, but it also means it has less stopping power and a shorter effective range.
(note: civilian SMGs are often converted that so they are only capable of semi-auto fire.)
Do not confuse it with assault rifle. An assault rifle is technically a cross between a conventional rifle and a SMG.
Uzi is commonly portrayed in movies.
The M1 Thompson, or "Tommy gun" is a submachine gun used during the Second World War.
Submachine guns are basically firearms that can be fired in single, burst, or automatic fire with equal ease over short to medium ranges, after which point they become less effective due to inaccuracy.
Submachine guns can actually be rather large, like an MP5SD or FN P90, or small, like a Mac-10. Having akimbo SMGs is becoming amazingly popular in action moves and video games. (though in realistic games and real-life, are not reccomended due to heavy recoil and inaccuracy)