A modem is an external box or internal circuitry that converts computer data into sound that can be transmitted over phone lines. First used to send telegrams, early modems alternated between two different tones. This is called modulation, and the process of modulating (and demodulating at the receiving end) gave the modem its name. These days modems transmit data with lots of different tones, signals, and complex mathematical processing, so modem is a bit of a misnomer.
