You have probably heard of printer ports and modem ports and perhaps ADB ports or some other kind of port. A port is a plug, or receptacle (known in other computer dictionaries as an input/output connector). Once you insert one end of a cable into a port, information can flow between your computer and whatever device is attached to the other end of the cable.
All ports, no matter how you connect to them, are either serial ports or parallel ports, which refers to the way the data flows through the wires (of course there are different cables for serial ports than for parallel ports). A parallel port accepts the cables that have parallel wires, so data can flow through the cable at high speed (in a serial port, data is transmitted in a single line). The special thing about a parallel port is that it can transfer a complete byte of information at a time. The port has eight data wires, one for each bit in the byte, so all eight bits can travel side-by-side and arrive at the same time. A serial port, by contrast, only lets one bit through at a time. All things being equal, a serial port is slower than a parallel port; however, a serial port can send and receive information at the same time, which many parallel ports can't do.
On pcs, parallel ports typically connect to printers, and serial ports to mice and modems. Here's the take-home message for PC users: if your computer and printer both have parallel ports, that's how you should connect them, unless they're farther apart than about ten feet. The longer a parallel cable gets, the greater the chance of "crosstalk," or interference, between data travelling on the two parallel wires. So if your printer and computer are fifteen or twenty feet apart, hook them up with a serial cable, or move them closer together and use parallel.
On Macintoshes, there are no Centronics parallel ports like those on pcs; modems are connected through serial ports, and printers are connected either through serial ports, LocalTalk network ports, or SCSI ports. LocalTalk and SCSI connections are usually faster than parallel printer ports, and are used for laser and color printers. Serial ports are slower and generally used to connect dot matrix printers.