I agree, there lots of genres, and whit in that 'main' genre there are dozens of sub-genres. It can get very confusing.
On my computer, when I get information from the internet it always give the genre simply as 'Rock' and I have to edit each song or album individually to the right sub genre.
For the style of music I listen to the main genre is "Metal", but there are dozens and dozens of sub genres (for example - death metal, thrash metal, black metal, Norwegian black metal, British metal, Power Metal, nu metal). I heard from a documentary [Metal: A Headbanger's Journey] that each individual sub genre has its own sounds, content of lyrics, the way the song is played I mean by vocals.
I noticed that people who don't like the kind of music I do [by genre in this case] just call it 'Metal' or 'Heavy Metal' they tell me that it just sounds the same, but for a fan like myself I can easily hear the differences. Well thats my opinion