Screaming is a vocal technique often used in punk rock, fast played metal music, and screamo. It is usually simply either a louder than normal high pitched form of singing, or a louder than normal low pitched form of singing. In both cases the vocalist usually tries to make it sound like some sort of beast's growl. For this reason, a low pitched scream is usually called a death growl.
More experimental forms of screaming are often used in alternative rock and emo music. These genres often have shrill screaming that has an emotional tone to it. The band, Thursday, is a good example of this.