In the past, most IBM PC and compatible computers were typically equipped with two serial ports and one parallel port. Although these two types of ports are used for communicating with external devices, they work in different ways.
A parallel port sends and receives data eight bits at a time over 8 separate wires. This allows data to be transferred very quickly; however, the cable required is bulkier because of the number of individual wires it must contain and cable distances are generally very short.
Parallel ports are typically used to connect a PC to a printer and are rarely used for much else. A serial port sends and receives data one bit at a time over one wire. While it takes eight times as long to transfer each byte of data this way, only a few wires are required. In fact, two-way (full duplex) communication is possible with only three separate wires - one to send, one to receive, and a common signal ground wire.