Question on using different busses

• Usually send all your tracks to the Master bus.  This is where you control your overall volume/output.
• You can set up a bus for an effect like reverb.  Each track that needs reverb is still output to the Master bus, but it also has a send that goes to the reverb bus.  (This is like a Send/Return on a traditional mixing console.)  This way your CPU only has to deal with one instance of reverb.  Even if you have lots of CPU power, though, a reverb bus makes it easy to have a consistent reverb sound.  It can be weird sounding if one singer is in a cathedral and one is in a small rehearsal hall.  You can also control the overall reverb level in your project more easily, or change the room size or delay time in one place instead of 20 places.  Note, however, that there is nothing wrong with putting separate reverbs on several or all tracks if you want to tweak the reverb for each track.
• I sometimes set up a vocal reverb bus and an instrument reverb bus.  Although this technically violates the rule about everyone being in the same space, sometimes vocals, to me, need different reverbs than instruments.  (I use way less reverb than many people so it will be interesting to hear what others say.)
• You can also use a bus to simplify mixing several tracks.  Example: You have a lot of harmony parts.  Instead of routing them to Master, route them to a bus called Harmonies and route that bus to Master.  Mix them individually the way you want and then use the fader on the Harmonies bus to mix in the overall amount of harmonies you want.  During your final mix, if you decide the harmonies sound great but are all a little too loud or quiet, you don't have to go mess with 12 faders and hope you moved them all the same amount.  This also works for drums or other things you want to group.  I just had a recent thread on Grouping that talks about this more.