For myself I don't feel a great need to categorise music, most of the time. For me, it's music, and if it moves me or excites me I tend to like it, and if it doesn't I don't. People ask me what kind of music I play as a quick way for them to make a snap decision about whether they'd like it or not - but I'd much prefer them to listen to it to make up their minds without a definition, as even if I said "Celtic and English but I write my own songs which tell stories and play guitar" that still doesn't actually convey the music. I like some music generally called classical, some blues, some country and western - but not all of any category.
Mostly the categories are used for music shops to put CDs into browsers, but wouldn't we all be enriched if Beethoven nestled next to the Beatles and Blowzabella?
As to World music, I've always been puzzled about why only certain geographic areas count as being in the world ...where are the rest of us?

Anne