Short answers, pulled from the story.
Heavy metal developed in the late 1960s and early 1970s, largely in the United Kingdom and United States. It grew from roots in blues rock, psychedelic rock, and acid rock. In 1968, three of its most famous pioneers, Led Zeppelin, Black Sabbath, and Deep Purple, were founded.
Black Sabbath's 1970 self-titled album is generally accepted as the first heavy metal album. Some also point to Blue Cheer's 1968 debut Vincebus Eruptum as the first true heavy metal recording.
Led Zeppelin, Black Sabbath, and Deep Purple, all founded in 1968, are the genre's most famous pioneers, sometimes called the unholy trinity. American bands like Alice Cooper, Kiss, Aerosmith, and Van Halen later modified metal into more accessible forms during the 1970s.
Heavy metal is traditionally characterized by loud, distorted guitars, emphatic rhythms, dense bass-and-drum sound, and vigorous vocals. The guitar power chord, extended guitar solos, and high volume are central, with the bass playing a more important role than in any other genre of rock.
Major subgenres include thrash metal, death metal, black metal, power metal, doom metal, and gothic metal, along with glam metal, groove metal, and nu metal. Thrash broke into the mainstream with Metallica, Slayer, Megadeth, and Anthrax, the Big Four of Thrash.
The origin of the term in a musical context is uncertain. Writer William S. Burroughs used Heavy Metal Kid in his 1961 novel The Soft Machine, and Steppenwolf's 1968 song "Born to Be Wild" gave the first use in a song lyric. Critic Lester Bangs of Creem is credited with popularizing the term in the early 1970s.
Heavy metal lyrics have been targeted as juvenile, misogynistic, or occult. In 1985 the Parents Music Resource Center petitioned Congress and Dee Snider defended his song at a Senate hearing. Ozzy Osbourne and Judas Priest were both sued over song lyrics, and metal fans have been arrested in countries including Egypt and Malaysia.