Sorry, I don't know why I said "hardcore punk." I meant to say "post hardcore (punk)" because I think that is what people refer to when they say "emo" or "screamo." In particular, "screamo" bands seem to love using harsh vocals with clean vocals, something that post hardcore bands do too. The other two seem to be terms that are used to describe the bands themselves, rather than their music.

And I disagree with the metal genres thing, to an extent. The instrumentation, vocals, and lyrics of most the varying metal genres are quite different and clearly defined. Things such as "speed metal" or "viking metal" are dumb, but metal actually does have a lot of legitimate subgenres.