Many people wonder what the difference is between metalcore and screamo. Many have the wrong idea.
Metalcore has roots in both hardcore punk and metal. When blended together, a subgenre of metal is created. Well, metalcore being metal is actually quite controversial, but that's not the point.
Screamo is a subgenre of emo, which also has roots in hardcore punk.
Both genres heavily feature screamed vocals. There is, however, quite a big difference. Screamo's riffs are more pop punk and emo based, hence it being a subgenre of emo. Metalcore features breakdowns, where part of the song, usually the chorus, is slowed down, and often clean vocals are featured. Lyrical themes are also quite different. Screamo's lyrics are more emotional, metalcore's are not, and are more diverse.
Of course, some bands just have to break the trend. Bullet For My Valentine are somewhere in between. Their music features a lot of breakdowns, and not a lot of screaming, but their lyrics are quite emo.
I read somewhere that Death is a metalcore band. That can't be further from the truth. Death is a death metal band - something that actually has practically nothing to do with metalcore OR screamo.
Bring Me The Horizon
Asking Alexandria (though some songs are metalcore)
Attack Attack! (same as Asking Alexandria though, some songs are more metalcore based)
Falling In Reverse
30 Seconds To Mars (though not all songs feature screaming)
Devil Wears Prada
August Burns Red
Some old AFI songs (for example, many tracks on Shut Your Mouth and Open Your Eyes could be called metalcore)
By no means are both lists definitive. They are just a small sample of the more well known bands that fall into the Screamo and Metalcore categories.
A typical screamo song:
A typical metalcore song: