Wikimedia
Wikipedia: Modem
Wikipedia provides an extensive encyclopedia definition of the term, "Modem," including history, description, internet access, and more.
Other
Learn the Net: About Modems
A basic overview and comparison of types of computer modems. Covers phone modems, DSL, cable modems, ISDN, and business T1 and T3 lines. Good animated illustration of data transmission.
Computer Hope
Computer Hope: Computer Modem Drivers
A useful chart containing computer modem from different companies. Also answers the question "How to determine what type of modem I have"?