Album Artist (Auto) : This is a calculated field based on the Album and Artist tags. If all of the Artists for an Album are the same, then Album Artist (Auto) will be the same as Artist. If all of the Artists are different, then Album Artist (Auto) will contain (Multiple Artists). If you create your View Schemes based on Album Artist (Auto) rather than just Artist, then they will show albums under (Multiple Artists) rather than individually. It's really handy for any kind of Album-centric View Scheme.
Album Artist : This is a manual override field for Album Artist (Auto). If you are happy with your compilation CD's showing up under (Multiple Artists) then you can leave this blank. If you'd rather specify your own "artist" for a specific CD (compilation or otherwise), then enter that in here. For instance, many of my compilation CDs are soundtrack albums. If I put "Soundtrack" into the Album Artist field, then these CD's will show up under the artist "Soundtrack", as long as I'm using a View Scheme based on Album Artist (Auto). If I leave this field blank, then they will show up as (Multiple Artists).
Multi-Disc CDs : What I do is pretty simple. I give all tracks the same album name, minus the (Disc 1), (Disc 2) stuff. For instance, Metallica's Garage Inc. is just "Garage Inc." and not "Garage Inc. (Disc 1)" and "Garage Inc. (Disc 2)". Then I just renumber the tracks accordingly. The only time I preserve the disc numbers in the Album tag is when there's something to gain by doing so. For instance, Metallica's Binge & Purge is 3 discs. Each disc is a different concert. So I keep the 3 discs seperate in order to better choose which concert to listen to.