One or more tracks could be used to make up a channel. For example one channel may record all drum information but there could be a number of tracks that contribute to this eg one for the bass drum, one for the snare drum, one for a cymbal. Why have various tracks? Because they can be edited to a larger degree than channels. That is within a drum kit you may want the cymbals to be louder (proportionately) than the bass drum. Another main reason professionals use tracks is to get a true stereo split, for example one drum could come out of the right speaker and another one the left.