Modem:  A card, which plugs into the bus slots on the mother board, that converts computer data to and from a format that can be sent over phone lines.  A standard home or business phone cord plugs into the modem (which extends out of a slot on the back of the computer).  A phone can be plugged into a second jack on the modem, so that you don't lose your current number of phone jacks (a phone is not required to operate the modem, the second jack just lets you plug in a phone near the computer). 

VIDEOS:  phone cable into modem back panel, modem install, modem install #2, phone plugged into modem

PHOTO:  Phone plug on modem

Modems are used to connect personal computers to Internet service providers, other computer services, and to any other distant computer that has a modem.  Using communications software such as Microsoft Internet Explorer and Netscape Communicator, you can dial the phone and send and receive data with the computer(s) at the other end.  Modems are most often used to download, or copy information and programs to your hard drive, communicate with email, and send and receive graphics, sound, and other types of information with other distant computer users.  Many modems also include the ability to send and receive faxes, which you can save on your hard drive, or print on a printer.  External modems can also be purchased.  They're usually connected to the serial port on the back of the computer.