Metalheads are people (both girls and guys)who enjoy and are particularly devoted to metal music. Metal music includes (limited list - theres LOTS more subgenres): death metal, black metal, heavy metal (for example 80s heavy metal), nu metal (often disregarded by old school/death metal lovers) symphonic/gothic metal etc.
Metal heads dont HAVE to dress in a particular way, however a STEREOTYPICAL male metalhead could be seen wearing long pants (ie army pants) in dark colours like grey, black, karki green etc, a band shirt often bought from a concert, and long hair - popular because of effect when headbanging. patches are also popular. Females dress is harder to stereotype - ranging from gothic tendancies to indie/emo (thrift store emo - not wannabe hate life emo).
some metal heads (particularly males) get very defensive when describing a metal head - often calling those not as devoted as them as posers (often assosiated with nu metal)