The word "modem" is short for "modulator-demodulator." The sending modem converts the digital data from a computer to an analog signal that can be sent over telephone lines; then, the receiving modem at the other end of the line demodulates the signal, converting it back into digital data.
Modems are generally sorted according to their throughput ? that is, how fast they can send data. This rate is expressed in bits per second (bps) or bytes per second (Bps). The first modems were introduced commercially around 1960 under the marketing name Data Phone and could transmit at up to 2,400 bps.