there are alot of genres that fall under the "emo" catagory...emo is bands like taking back sunday, the used, and dashboard confessional, these bands don't scream all that much. as for the look it usually involves guys wearing girls pants, longer hair over your eyes, and a great deal of tight black shirts.
screamo is another genre, but I guess could be a sub genre of emo... bands in this genre often scream more then sing, examples being underoath, and alexisonfire.
Believe it or not the music is not terrible, im a big fan of Alexisonfire and Taking back sunday.. hope this cleared it up a little more for anyone.