As stated sub machine guns although not necessarily light are easily caried by one man and initially fired pistol caliber rounds (often somewhat hotter than you would want to fire from a pistol however). Machine guns can be put into several different categories:

Light machine guns: rifle caliber guns that can be carried by one man and fired on the move although the preference is to use a bipod or tripod. May be belt or magazine fed. Examples would be the Bren historically or the SAW in modern times. Examples would be the Thompson, Sten, Uzi, etc. Smaller ones are sometimes called machine pistols.

Medium machine guns: Usually around 8mm rifle caliber rounds (in WWII and earlier often the same as used by service rifles and LMGs). Usually belt fed and operated from tripod. Not easily fired on the move unless vehicle mounted. Sometimes water cooled. Higher sustained rate of fire than LMGs.

Heavy Machine guns: Usually 12-20 mm although occasionally water cooled ~8mm guns are included in this. Fired from a mount or tripod. Examples include various 12.7mm guns, the Soviet 14.7, German 15mm, etc.