Take some of the lighter elements of punk, some of the lighter elements of goth, and some elements of "rocker" styles. Mix em all up, match it with commercialized "pseudo punk" music, and add a dash of teenage angst and that's "emo".
This sums it up:
Personally I don't have a problem with any of it. It's just teens being teens. In my day, it was those long-sleeve rocker shirts, super-tight jeans, rippe jean jackets and hair with tons of hair spray. Same ---steaming pile of meadow muffin---, different look. Of course, adults have short memories, so....