Wikimedia
Wikipedia: Polyphony
Wikipedia provides a brief description and history of the term, polyphony, including many hyperlinked terms.
Other
Thirteenth Century Polyphony
Detailed article about polyphonic music and its components in the 13th century.
Stanford University
Stanford University: Medieval Music Birth of Polyphony
An article on the beginning of Polyphonic music during the Middle Ages. You'll also find illustrations.