The heavy metal (literally "heavy metal", often shortened to metal) is a genre of rock music. Resulting from hard rock, is characterized by highly aggressive rhythms and powerful sound, achieved through the emphasis of the amplification and distortion of the guitars, bass, and often even the voices. The musical themes are often referred to as a dream, angry or violent.There are a multitude of styles and subgenres of heavy metal. Consequently, there are subgenres more melodic and commercial, and other extreme sounds and underground.Already very popular in the seventies and eighties, he continued to have success in the following decades and has also diversified into numerous subgenres. Many artists and groups are ascribed to metal, both in musical mechanisms official within the underground.