Understanding iTunes Music Folder Files

Can someone please help me make sense out of the clutter that has become of my iTunes Music Folder?

Please see the screenshot below. I am quite certain that there is abunch of archival /legacy files within this folder, as well as a bunch of stuff that doesn't really need to be in this folder any longer, and I would like to know what is what so that I can begin cleaning things up a bit.

Also when I make backups of my iTunes Library (minus the audio/video files) which files are the ones that need to be backed up?



and the expanded view:



...any help would be greatly appreciated! Thanks!