One of the best genres of metal there is in my opinion. Originated in the late 80's. Cross between hardcore metal and hardcore punk. A lot better than shitty nu-metal.
Metalcore bands include the OLD avenged sevenfold, OLD atreyu, underoath, lamb of god, killswitch engage, bullet for my valentine, as i lay dying, some black dahlia murder, all that remains, some machine head are some of the more mainstream stuff. a good unknown band is august burns red.