Metalhead is a label for someone who listens to metal, any kind of metal. Heavy metal, hair metal, power metal, progressive metal, thrash metal, death metal, black metal, gothic metal, metalcore, grindcore, industrial metal, doom metal, sludge- hell, even nu-metal. Some people will dispute that last one of course. Metalhead is a label for a fan of one of the most subgenre-obsessed and purity-obsessed musical subcultures in existence.
An active metalhead can probably give you several different definitions of True Metal, at least one of which involves spelling it as Trve Metal, a list of genres descending to the sub-sub-genre level of classification (Ex: Jazz Influenced Technical Death Metal), a discourse on how genres differ according to country (Ex: US black metal vs. Norwegian black metal), an explanation of the "Horns", and an interesting debate over if listening to the music of a band requires supporting the philosophies of that band. Anyone with an interest in black metal will have had to deal with Satanism as well as the more ominous National Socialism and there are metalheads to span the entire range of possible reactions to these philosophies including "embrace with open arms". Of course most metalheads are just there to enjoy the music and maybe bang their heads around some.
If you want to learn more about metalheads, you may be interested in the following productions:
Metal: A Headbanger's Journey
This is Spinal Tap
The Decline of Western Civilization Part II: The Metal Years