The box shaped part of a computer is called as system unit of main computer.The system unit itself is the main computer.It houses motherboard, all the circuit boards, hard disk, floppy disk drive, tape drive on/off switches sockets for connecting monitor, printer, mouse, modem and other peripherals to computers.System unit comes in two flavours a.the desktop and the tower type.
In the past, the desktop type system unit was very popular.But with the passage of time, it lost is popularity, and now tower type system unit is generally preferred by buyers of pc.