Main Database:
~d=mLike it says on the tin.
CD\DVD database:
~d=cAgain, like it says on the tin. Massively underused database imvho. Now completely hidden on a default installation. It catalogues every CD or DVD it encounters in your optical drives. You can associate coverart, change tags etc. etc. in the CD\DVD database just as you can in the main database. Remember, none of the files are actually imported, just a record kept. 4 years ago, I sat and fed my audio cds into my optical drives one after the other, MC did it's YADB thing, I helped it with those it didn't get, and to this day, all I have to do is put a cd in the drive and it's good to rip or play instantly. If I made any changes or tweaks to the CD's tags these are also refelcted in the resultant ripped file.
Build a viewscheme that combines the Main and CD\DVD databases to track online and offline media in the one place. Mine used to load quite slowly, but with this latest build (110) it's super fast.
Playing Now database:
~d=tKeeps track of everything added to playing now during the current session. This database is cleared when MC is restarted.
Explorer Database:
~d=eKeeps track of files browsed using the "Drives and devices > My Computer" branch of the tree. Again, this database is cleared when MC is restarted.