Folk Metal is a genre of music that blends together the best of heavy metal and European folk music
Folk Metal is an... interesting genre of music. It's unique insofar as no other genre of music contains both pentatonic guitar solos and folk interludes, usually in the same song. Nearly all folk metal bands use one theme in their music, which sets them apart from other bands.
It takes a special kind of person to be in a folk metal band. They have to love the edge of metal while also have an appreciation for medieval settings. So in essence, nearly all folk metal bands seem to be composed of disgruntled Renaissance Fair employees.
Welcome to Ye Olde Renaissance Fair. Have a corndog.
Of course, there are a few distinctions.
Viking metal is fairly self-explanatory. It's about vikings. Keeping with the tradition of Wagnerian opera, viking metal often sounds large and epic, complete with screeching guitars and pounding drums set to expansive keyboard/orchestra arrangements. Most of all, viking metal bands love Norse mythology, which is generally considered the most metal mysticism in known history.
Let there be rock.
Celtic metal, much like viking metal, is characterized by loud guitars and mythology, but this time it's Celtic (Irish) mythos. It's understandable why a metal band would be fascinated with this. After all, a central figure in Celtic mythology is Cuchulainn, a teenage boy who was pretty much today's equivalent of Wolverine. At the age of 17, he singlehandedly slaughtered an army trying to invade his homeland, presumably because he had U2 tickets, and the opposite army was in the way.
Sunday Bloody Sunday motherfuckers!
Out of all the variants of folk metal in the world right now, this one is the most left-field. Pirate metal's genesis came in the late 80's when the German metal band Running Wild released the album Under Jolly Roger, but it wasn't until 2005 that two bands emerged as pirate metal torch bearers: Swashbuckle and Alestorm.
Swashbuckle in particular like the pirate metal genre. All their songs concern things like drinking and pillaging, all the while dressed like real life pirates.
Alestorm is similar, except they're reminiscent of Iron Maiden with accordions. Most likely it's because their unofficial mascot is a rotting, dead pirate, much like Iron Maiden's rotting, dead zombie friend.
Oh hi Eddie, I didn't see you there