In computing, a serial port is an interface on a computer system with which information is transferred in or out one bit at a time (contrast parallel port). Throughout most of the history of personal computers, this was accomplished using the RS-232 standard over simple cables connecting the computer to a device such as a terminal or modem. Mice, keyboards, and other devices were also often connected this way.
While RS-232 originally specified a 25-pin D-type connector, these were large and awkward, and most of the pins were unused (after all, since data is sent one bit at a time, only one wire is needed for data in each direction plus a few control signals), so it was common to use other connectors for these ports (in particular the 9-pin version used by the original IBM PC). In Europe, the related RS-422 standard was popular, and often used German DIN connectors.
In very recent years, more advanced electronics has made possible more reliable and higher-speed serial communications, so RS-232 is being supplanted by newer standards such as USB and Firewire. These make it possible to connect devices that would not have been feasible over slower serial connections, such as storage devices and sound and video devices.