Data from the computer in the form of digital signal provided to the modem to be converted into an analog signal. Analog signals can be transmitted through several media, telecommunications such as telephone and radio.
Arriving at the destination modem, the analog signal is converted into digital signals back and sent to the computer. There are two types of modems are physical, ie an external modem and internal modem.
Source : http://id.wikipedia.org/wiki/Modem