An automatic (rapidfire) weapon first developed in the late 19th century. The recoil from the first bullet fired resets the mechanism to fire again. The next bullet is pushed into the barrel from the magazine - either by gravity (as in early weapons) or a tensed spring. Revolutionised warfare by making it notoriously easy to mow down large numbers of attacking infantry at decent range, was very handy when conquering spear-armed peoples. Went on to a much longer and bloodier history as the design was refined.
Comes in a variety of calibres, weights and rates of fire, from sub-machine guns (use pistol bullets to reduce size and weight) to light machine guns (use assault rifle bullets for compatability and to reduce size and weight) and heavy machine guns (sheer firepower, no compromises).
SMGs and LMGs are meant to be carried. Anything larger, and you're doing a Rambo.
The M2 machine gun, the original and the best. Used by the US from WWII right through to now. Ever wondered why the MGs on the top of Sherman and Abrams tanks look so similar? They're M2s.