Balancing tracks
I don't want the vocals to be overpowered by drums, but at the same time, I don't want the listener to have to strain to hear the instruments... it's a balancing act. Not to mention the fact that the instruments should be prominent during instrumental breaks; making the volume changes sound gradual and natural is very challenging, surprisingly.
Mixing tracks
Mixing tracks
Balancing and mixing tracks are very similar -- and some people consider them one in the same, in fact. Really, though, when I am mixing tracks, I am playing with panning (one of my new favorite things to incorporate into the tracks!)