Using Aux busses does add a bit of complexity to the way you mix.
There are two primary things to grasp here, send and return. You control the send (the amount of signal to send from a track) using the Aux Send fader contained in each track mixer, and you control the return (the level the aux channel will return to the master bus) using the Aux Return faders, which are at the top of the main mixer to the left of all the aux channels.
How you use the aux channels really depends on what you want to do.
For example, let's say you want to apply the same reverb to 5 tracks. One way to do this is to put the reverb on the aux channel, and adjust it's wet/dry settings until they are exactly what you want. Then you can set the return from that aux channel to 0db (100%), configure its sends to be pre-fader, set each track volume slider to -inf (0%), and control the level of each track using the Aux send slider on each track.
Another way to set that up is to put your reverb on an aux channel completely wet (e.g. all reverb, no dry sound mixed in), with a return level of 100% and its sends as post-fader. Then you would set the levels of each track normally, and control the amount of reverb per track using the aux sends.
Yet another way to set that up is to put your reverb on an aux channel completely wet, set the levels of each track normally, set the send to aux on each track to 100%, and use the aux return to control the reverb applied to the combination of all 5 tracks.
...and so on and so forth. Note that all three of these examples, while similar in nature, will produce different sounding results!
Also note that the aux channels can each be configured to be pre-fader, post-fader, or pre-insert, by clicking the "P" button underneath each aux channel in the main mixer.
pre-fader = the signal is sent to the aux bus BEFORE the track's volume fader and effects (also pan...whenever I say volume, I could also be saying pan because they are processed together in n-track)
post-fader = the signal is sent to the aux bus AFTER the track's volume fader and effects.
pre-insert = the signal is sent to the aux bus AFTER the volume fader but BEFORE the track's effects are applied.
Cripes, lots of stuff to digest. Try thinking about things as if you were using a real mixer with real effects, because that is what all these options are replicating. Once you get the swing of it, you'll understand both the creative freedom and ease of mixing properties of the aux channels.
Good luck, and sorry my descriptions are so poor tonight, but it's pretty late...
Slackmaster 2000