Hard rock (or heavy rock) is a loosely defined subgenre of rock music which has its earliest roots in mid-1960s garage rock, blues rock and psychedelic rock. It is typified by a heavy use of aggressive vocals, distorted electric guitars, bass guitar, drums, and often accompanied with pianos and keyboards.
Hard rock developed into a major form of popular music in the 1970s, with bands such as Led Zeppelin, The Who, Deep Purple, Aerosmith and AC/DC. It reached a commercial peak in the 1980s, with glam metal bands like Van Halen, Bon Jovi and Def Leppard and the rawer sounds of Guns N’ Roses, which followed up with great success in the later part of that decade, before losing popularity with the commercial success of grunge and later Britpop in the 1990s. Despite this, many post-grunge bands adopted a hard rock sound and in the 2000s there came a renewed interest in established bands, attempts at a revival, and new hard rock bands that emerged from the garage rock and post-punk revival scenes.
(Wikipedia contributors. “Hard rock.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 23 Jun. 2014. Web. 13 Jul. 2014.)