Short answers, pulled from the story.
Hard rock is a heavier subgenre of rock music typified by aggressive and distorted vocals and electric guitars. The electric guitar is central, used both as a rhythm instrument with repetitive riffs and as a solo lead, while drumming focuses on driving rhythms with a strong bass drum and snare backbeat. Vocals often range from growling or raspy to screaming, wailing, or falsetto.
Hard rock began in the mid-1960s with the garage, psychedelic, and blues rock movements, drawing on electric blues traditions from the mid-to-late 1950s. Among the earliest hard rock recordings were works by the Kinks, the Who, the Rolling Stones, Cream, and the Jimi Hendrix Experience. Blue Cheer, Led Zeppelin, Iron Butterfly, and Deep Purple also produced hard rock by the late 1960s.
In the late 1960s the terms were used interchangeably, but hard rock maintained a bluesy rock and roll identity with swing in the backbeat and riffs that outlined chord progressions, while heavy metal riffs functioned as stand-alone melodies with no swing. In the 1980s, heavy metal developed extreme subgenres influenced by hardcore punk, further separating the two styles, though bands frequently crossed the boundary.
Guns N' Roses released Appetite for Destruction in 1987, the best-selling debut album of all time. It featured a grittier and rawer sound than most glam metal of the era and produced three hit singles, including "Sweet Child O' Mine."
Nirvana's Nevermind in 1991 combined hardcore punk and heavy metal into a sound with darker lyrical themes that displaced glam metal. Grunge bands rejected the macho and fashion-focused aesthetics of hard rock, and glam metal bands like Europe, Ratt, White Lion, and Cinderella broke up as commercial momentum shifted. Post-grunge acts later adopted heavy guitar sounds in a more radio-friendly format.
From the 25th of June to the 5th of November 1988, hard rock albums held the Billboard 200 number one spot for 18 out of 20 consecutive weeks. The albums in rotation were OU812 by Van Halen, Hysteria by Def Leppard, Appetite for Destruction by Guns N' Roses, and New Jersey by Bon Jovi.