To the uninitiated, any loud music is called heavy metal. In reality there are a multitude of heavy metal styles and subgenres. Heavy metal is a wide umbrella characterizing a style of music that is generally loud and aggressive. There are genres that are very melodic and mainstream, and other genres that are extreme and underground. Here's a brief overview of heavy metal and its many styles. The term "heavy metal" was first used in a musical sense in the '60s song "Born To Be Wild" by Steppenwolf when they referred to "heavy metal thunder." Although there are debates among experts, most consider groups like Black Sabbath, Led Zeppelin and Deep Purple to be the first heavy metal bands. From there the style evolved and branched into many different genres and subgenres. Heavy metal remains a vital force in music today, with sellout concert tours and CDs selling impressive numbers of copies without any radio airplay or MTV exposure.
Musical and Vocal Styles
The backbone of heavy metal is the electric guitar. You can't have metal without at least one guitarist, and many bands have two or more. Certain genres have some quiet and mellow parts, but most metal is loud, intense, fast and aggressive. The vocal styles in heavy metal range from melodic singing to aggressive singing to unintelligible screaming, depending on the genre.
At the beginning there was just traditional heavy metal. Shortly after it evolved and splintered into many different styles and subgenres. This site has a series of articles on many of the genres that will give you a more in-depth look at that particular type of metal. As time has gone on, there are literally hundreds of subgenres, but these are some of the main genres of heavy metal:
Avant Garde Metal
Also called experimental metal, it's characterized by unusual and nontraditional instruments and song structures. Examples: Arcturus, Dog Fashion Disco, Mr. Bungle, Peccatum, Vintersorg