A modem is used to link a computer to the telephone system, allowing data to be transferred to it and from it and thus to be sent and received. A modem can be internal (part of the "box" part of the computer) or external (linked to the central processing unit by a lead).

"Modem" is short for "modulator/demodulator", meaning that the modem modulates the digital data produced by the computer to make it compatible with the analogue signals carried by the telephone system before sending it, and demodulates the data which it receives

(Cotton, Bob, and Oliver, Richard, The Cyberspace Lexicon, Phaidon Press, London, 1994, p134).