A modem is a device that connects a personal computer to a large computer (called "a host computer") through regular telephone lines. When a host computer is connected to the Internet, various services such as e-mail and the World Wide Web become available to a personal computer. The single most important factor of a modem is its speed. The speed of a modem is expressed in "bps" or "bits per second." The current popular speed is 56K bps (56000 bps). Other slower speeds are 33.6K bps, 28.8 K bps, 14.4 K bps and 2400 bps. Modems are available for internal installation (a circuit board to be installed inside a computer) and for external installation.