Serial ports are 9-pin connectors that transmit incoming and outgoing information - one bit at a time. Throughout most of the history of personal computers serial ports were used to transfer the data from computers to various devices. Before internal modems, there were external ones and they were connected to computers via serial ports, aka communication or COM ports. The same with mice and keyboards.

Serial ports are controlled by a special chip - UART (Universal Asynchronous Receiver Transmitter). UARTs are serial chips on the computer's motherboard (or on an internal modem card). On older computers like many 486's, the chips were on the disk IO controller card.

When all computers had parallel bus architecture, the UART's role was to convert bytes from the PC's parallel bus to a serial bit-stream.

As USB ports are much more spread at the moment, modern computers are widely manufactured without serial ports. Such computers may require serial-to-USB converters to allow compatibility with RS-232 devices, because serial ports are still used in industrial automation systems, some scientific instruments and so on.