The word modem is an acronym for modulator-demodulator. Its name describes its function. A modem is a device or program that enables computers to send data over telephone lines. Computer data is digital, capable of distinguishing between zero and one. The opposite of digital is analog--the infinite smooth gradation between numbers. A modem converts the data supplied by a computer in digital form to a modulated analog waveform that is able to travel over telephone lines. A modem can also work in reverse--accepting an incoming modulated analog wave and converting it to digital form, which can then be passed on to a computer. A modem is thus the communication bridge between computers.

Modems can be located inside a computer, or can be an external attachment. An internal, or onboard, modem is a type of printed circuit board called an expansion card. An external modem is a piece of...