A question

I have a zillion MP3s.
OK, only about 3000, but for ME that’s a lot.
Anyway, I really want to reclassify their genres. I’ve been puttin it off but it’s starting to really irk me the way I have my genres set up (basically, I only have about a dozen genres and all my MP3s are assigned to one of ’em, but some are too general and some are too specific). I would really like to hear some folks talk about classifying music. I can tell really obvious genres apart (say, classical vs. heavy metal) but some seem to overlap so much that I’m not sure how to tell what genre a track belongs to. What’s the difference between trance, house, techno, and electronica? How do I tell if a song that’s somewhere near the middle of the blues/rock spectrum is blues or rock?
I really wish I could assign multiple genres to one MP3, but since that’s not yet possible, can someone offer some advice?