DevilsSon is correct - it's a good explanation.

Some history though - back when Led Zeppelin first became popular and then when they became very big - they were known (and billed) as heavy metal. This was in the seventies though and I think the meaning changed a lot in the eighties. Before that - Iron Butterfly was know as heavy metal. Has anyone ever heard In-A-Gadda-Da-Vida? At the time some people called that psychedelic music in addition to heavy metal.

Kathleen