Fancy words for counterpoint - two melodies at the same time. Polyphony is a term used in contrast to monophony which is just one melody at a time without any accompaniment, homophony which is when a melody is matched with an accompaniment that is secondary to it, heterophony which is when variations of the melody are all played at once, and polyphony which is when multiple equally important melodies are played at once. And canonic imitation is when melodies imitate one another as if in a canon or a fugue (although usually not as strictly).