The different between the two is relatively simple, monophonic music has a single voice, or group of voices all performing the same melody with the same rhythm. Examples of this are easily found in Gregorian Chant, here's a random example from youtube: http://www.youtube.com/watch?v=sYO2NPOHf0w
Polyphonic music consists of more than one melody occurring at the same time in a single piece. Fugues and rounds are good examples of this. Here's an example of a fugue by Bach which is polyphonic: http://www.youtube.com/watch?v=6clTa8_QYQE
Also, an article that explains the difference between polyphony and monophony in a little more detail: http://www.aboutmusictheory.com/music-texture.html
Polyphonic music consists of more than one melody occurring at the same time in a single piece. Fugues and rounds are good examples of this. Here's an example of a fugue by Bach which is polyphonic: http://www.youtube.com/watch?v=6clTa8_QYQE
Also, an article that explains the difference between polyphony and monophony in a little more detail: http://www.aboutmusictheory.com/music-texture.html