Once upon a time music was monophonic. The singer sang the song. The musician on the ancient, still very primitive tool sang one tune. But centuries passed, and the music was complicated. People realized that you can sing two or three, and not one and the same melody, but different, but merging together, forming together a pleasant for hearing harmonies. What you can play, too, not one, but several musicians together, not the same, and differ from each other ringtones.
Polyphony has two main types. If the leading value has only one voice, which sounds the melody, and the others accompany him, we talk about homophone (the Greek word homos means equal, phone - sound, and all together - unison, the same sound) or homophonic-harmonic style. This style is written most of the works of the XVII-XIX century and contemporary music.
Another type of polyphony have the widest distribution in the XVI - XVII centuries and reached its Zenith in the works of Johann Sebastian Bach. This polyphony (Greek poly - many, phone - voice, sound). In polyphonic pieces all voices are my own and are equally important, equally expressive melodies.