The word modem, a blend of "modulator" and "demodulator", refers to a device that modulates an analog "carrier" signal (such as sounds over a telephone) to encode digital information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Primarily used to communicate via telephone lines, modems can be used over any means of transmitting analog signals, from driven diodes to radio.
Early modems used frequency shift keying and simple frequency division duplexing. Later modems used first phase shift keying and then quadrature amplitude modulation with echo cancellation to give progressively higher performance.
ADSL modems are also a kind of modem, the main difference being that they are not limited to audio frequencies over the telephone line. Recent ADSL modems use coded orthogonal frequency division modulation.
See modulation for a fuller list of modulation techniques.
Modems are the most popular means of Internet access, UCLA 2001 study of American Internet users shows that 81.3% of them use telephone modem, and 11.5% cable modem, an order of magnitude more than any other method.