Polyphony means “different sounds or voices”. Polyphonic music has parts that weave in and out of each other. Polyphonic music is also sometimes called contrapuntal music. Polyphonic music may contain an element of imitation , where one voice or instrument copies what has just been played by another (think of a “round” like London's Burning.)
Imitation is where a melody in one part is repeated a few notes later in a different part, overlapping the melody in the first part which continues. For example, a flute may imitate a tune just played by the oboe.
Sometimes imitations contain slight changes to the tune to make it more interesting. These changes might be:
A fugue is a special type of polyphonic texture. Fugues always begin with a tune that is played on a solo instrument/sung by a solo voice or produced by instruments/voices in unison. This tune is then played by all the other instruments or voices in turn, but not necessarily at the same pitch.