"A number of heavy metal genres have developed since the emergence of heavy metal (often shortened to metal) during the late 1960s and early 1970s. At times, heavy metal genres may overlap or are difficult to distinguish, but they can be identified by a number of traits. They may differ in terms of instrumentation, tempo, song structure, vocal style, lyrics, guitar playing style, drumming style, and so on."
Credits go to Wikipedia and various other sites.
