There are two common methods of connecting a mouse to a PC. You can either use a PS/2 mouse port or a serial port. The PS/2 mouse port was introduced with the IBM PS/2 range of computers in the late 1980's. It takes the form of a small round socket.
Before the PS/2 computer came out, people used to connect their mouse through a serial port and this remains the only option if you don't have a PS/2 mouse connector on your machine.
Some high-end mice dispense with the cable attaching them to the computer, and communicate using the same type of infra-red found in remote controls............