A metalhead is any fan of metal music. That's it. There are many different kinds of metalhead, just like there are many different types of metal. Most metalheads, despite what genre they like, are classified by the mainstream as "wierd," "scary," or even "emo goth posers." Note: Metalheads are not emo or anything like it, except for metalcore listeners who don't seem to know the difference. If you go up to most metalheads and talk to them, you may find that they are intelligent people. However, as with all groups of people, there will always be some that are... less-than desirable. These people should NOT be the basis for judging metalheads, just like terrorists should not be the basis for judging muslims, nazis should not be the basis for judging Germans, rednecks should not be the basis for judging Americans, etc. Most metalheads are generally misunderstood and just want to listen to the music they are passionate about, and have a good fuckin' time.
Joey the metalhead: A 15 year old who discovered bands like Iron Maiden and Slayer, and mostly listens to the old stuff. He also enjoys some new stuff, like Mastodon.
Kevin the Metalhead: Listens to mainstream, modern metal, such as Slipknot and Avenged Sevenfold. Is often discriminated against by other metalheads for liking that music, and doesn't seem to know the difference between metal and emo.
John the metalhead: 35 years old, with a family. He listened to metal as a kid, and while his hair might not be as long as it used to be, and he dresses more "average," he still loves to listen to bands like Judas Priest and Megadeth.
Then there's Jake the metalhead. Jake thinks that ALL metal is awesome, and listens to Heavy Metal, Thrash Metal, Power Metal, Death Metal, Melodic Death Metal, Groove metal, Black Metal, metalcore, and Nu-metal. He likes basically any kind of metal he can get his hand on, and rejects no genre of metal. He listens to Iron Maiden, Sabaton, Judas Priest, Slayer, Amon Amarth, All That Remains, Black Sabbath, Hammerfall, Emperor, Megadeth, Venom, Iced Earth, Slipknot, Testament, Morbid Angel, Exodus, Lamb of God, Mastodon, Rush, Cannibal Corpse, White Zombie, and any other band that makes the beautiful sound known as metal.