Most basically, a fan of metal
music. Metalheads are stereotyped as loud, obnoxious, stupid, drunk, and non-achieving; also as defiant, free-thinking, tough, and fed up with the social status quo. There is an element of truth to all of these, particularly the last.
Metalheads tend to be clannish, or at least identify with each other due to familiarity with esoteric bits of information on bands that most people have no clue exist.
Ways to spot a metalhead: long hair, t-shirts with band slogans and logos, denim or leather clothing, proclivity for yelling "JUDAS FUCKING PRIEST!" or other such barbarities.
Bands considered to (lyrically) sum up the spirit of heavy metal: Judas Priest, W.A.S.P., Deströyer 666, Exodus.