The army rank of Captain is an officer rank historically corresponding to command of a company of soldiers. The rank is also used by some air forces and marine forces. Today a captain is typically the commander, or second in command, of a company or squadron (NB: in the U.S. Army, a squadron is a larger, battalion-equivalent, unit).
Captain is one rank above a lieutenant (or first lieutenant) and one below a major (or commandant).
The rank of captain should not be confused with the naval rank of captain or with the commonwealth air force rank of Group Captain, both of which are more senior.