A carbine is a lighter, shorter type of rifle. The smaller size and lighter weight of carbines makes them easier to handle. They are often issued to high-mobility troops such as special-ops soldiers and paratroopers. Also they can be used by mounted or other non-infantry personnel who do not need full-sized rifles.
The U.S. M4 carbine is a classic example. Continuous-fire versions may also be described as submachine guns