Ports :Serial, USB and Parallel
Still present on the back of most personal computers, the parallel port was mainly used to connect computers to a printer with a thick unwieldy cable. In fact it was commonly called the 'printer port'. It looks like a long blue socket with 25 pin-holes arranged in two rows. The parallel port is still useful though for control purposes. For example, computer controlled devices can be connected via the parallel port. Control software sends electrical signals through the parallel port.
A USB port is a standard cable connection interface on personal computers and consumer electronics. USB ports allow stand-alone electronic devices to be connected via cables to a computer (or to each other).
A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers.
Serial ports, also called COM ports are used to connect devices such as modems via a serial interface connection means that a data bit in the order of one second. It is located on the back of a PC.