The modem is technically called the modulator-demodulator. It is an electronic device which helps transmit programs and data locally or around the world through the telephone line. The main function of a modem is to transform digital signals in to analog signals and vice versa.
With a modem you can subscribe to a growing number of online and bulletin board services or communicate with other computers located elsewhere which have modems attached to them.
A modem may be a card mounted inside the PC or it may be a separate piece of equipment that connects to the serial port of PC via a cable. Telephone lines that connect the modem to the telephone service.