Metalcore is a genre of rock music that fuses hardcore punk and heavy metal. It emphasizes mostly heavy metal elements in addition to slow, intense breakdowns. Its influences include bands such as Black Flag, Black Sabbath, the Exploited, Corrosion of Conformity, Suicide Tendencies, Slayer, Agnostic Front, and Motorhead, just to name a few. One of the first bands to be considered a metalcore band was Biohazard.
Vocally, the music is a combination of screaming and clean during the chorus of a song while. Musical elements include double bass drumming, dropped guitar tunings, and harmonized guitar riffs.
Some of the bands that are included in this genre include Killswitch Engage, Underoath, All That Remains, As I Lay Dying, The Devil Wears Prada, Avenged Sevenfold, Beloved, Haste the Day, Demon Hunter, Lamb of God, Norma Jean, and Zao.
Here are some links where I was able to find some information about this genre: