Also, I'm a bit confused as to how I would record over someone else's track......where would I plug in my guitar.......
This is the beauty of "multi track recording"
Your Boss BR1600 is essentially a standalone "16 Track" recorder
This means you can record a guitar on Track 1, a bass on Track 2, a kick drum on Track 3, a snare on Track 4, hi hats on Track 5, vocals on Track 6 etc. etc.
If you want to record a STEREO source you need 2 Tracks (one for the left channel and one for the right channel)
You are then able to blend the different tracks together (by changing their levels, eq, panning, apply fx etc.) to make your "MIX"
From reading a quick review of the BR1600 it appears that a computer based DAW (Digital Audio Worksation) such as Sonar, Cubase or Reaper will give you similar functionality within its software - but with MUCH (much, much, much) more flexibility
By collaborating you just put the other person's music onto 1 track (or 2 tracks if it is stereo) and then record your bit on another track - the key is then sending back just the new track (stem) to the person that is putting the finished song together
It is VERY confusing to start with but does get easier the more you do it
I would recommend picking a DAW (it is not secret I am a big fan of SONAR) and learning it inside out - many of the principles are the same across different types so they are all much of a muchness!
Good luck!