A modem is a device that converts the digital signals used by computers into the analogue form required for transmission over phone lines, cable networks or radio links and vice versa. Its name comes from combining the words modulator and demodulator. When you send data it modulates a stream of bits into variations of voltage, frequency or light intensity; when data arrives it demodulates those signals back into digital form so your computer can process them.
Different types of modem serve different access technologies. A dial-up modem works over a traditional telephone line, while a DSL modem connects via your phone wiring to deliver broadband. Cable modems use the same coaxial cables as television, and fibre-modem units (often called optical network terminals) translate light pulses from fibre-optic cables into electrical signals. In a typical home setup, modern routers combine a built-in modem with Wi-Fi and switching functions, so that one box both handles the signal conversion and shares the internet connection with several devices.