A Modulator-Demodulator, commonly known as a modem, is a device that converts digital data from a computer into an analog signal for transmission over telephone lines or other communication mediums. This process is called modulation. The modem also performs the reverse function, converting incoming analog signals back into digital data, known as demodulation. Modems are essential for internet connectivity, allowing computers to communicate over long distances. They can be found in various forms, including DSL modems, cable modems, and fiber-optic modems, each designed for specific types of internet connections.