Screaming. We did it as infants when we were hungry or wanted to be held. We did it as teenagers when we weren't allowed to go to a keg party because we were grounded for....well, going to a keg party. We do it now as adults when we get our paychecks and see how much money has been taken out for taxes. In the metal music world, screaming has it's own sound though. Metal was originally coined as a music term mostly because it's music is electrified in the metallic sound of the guitars and the over the top bashing of metal cymbals, but now it's become more than the literal.
Metal heads all over the world consider metal to be a music style that embraces an amplifiers volume going as high as possible without ear splitting feedback. Not only speed in the rhythm, but accurate speed with fingers flying over the guitar strings and double bass pedals sounding like popcorn popping on beat. And though the 80's hair bands we know and love were in their time considered true metal, they are now pushed toward the category of classic rock.
Metal music today is no longer just power chords and vocal melody, it's almost reversed. The melody comes from the band and mostly from the lead guitarist(s), and the power comes from the vocals of the singer who more often than not sings a little and screams a lot more. It would depend on a person's musical taste of course whether or not they prefer a band who screams the entire time or balances it out with some voice melody, but metal is now known for having this element and it is pretty much necessary to have if a band is going to categorize themselves as metal in today's music land. Screaming sounds range from the gutteral growling of black and death metal bands like Poland's Behemoth to the consistent melodic screaming of Finnish band Children of Bodom to bands that are more commonly balancing both singing and screaming such as Avenged Sevenfold and In Flames.
Screaming improperly can damage a singer's vocal chords so if you are the amateur singer who dreams of taking their place on the stage for the glory of all that is awesome or if you want to start a band just to get a date with a metal loving beauty queen, be sure to do your research beforehand. Screaming properly doesn't come from the vocal chords, it comes from the diaphragm. Pick up the Zen of Screaming DVD, call a vocal instructor who specializes in this kind of music, or at the very least look on-line for some tips. Otherwise, you'll sound like you're going through puberty all over again from stressing out your throat. There's nothing metal about cracking your voice when you are telling the audience to get their f***in horns up.