A modem is a device which converts computer information into sound. The sound can then be moved over regular telephone lines.
Modems enable computers to be connected so they can exchange information.
Both computers must have a modem. One computer sends its information to the modem which converts it into sound and sends the sound over the telephone. The other computer's modem listens to the sound, converts it back into computer data, and sends it to its computer. Thus, even though sound is moving across the telephone wire, the computers on each end of the connection only see computer data. Modems can transmit computer information around the world.