Hard Rock

Hard rock is a variation of rock and roll music which has its earliest roots in mid-1960s garage and psychedelic rock. It is typified by a heavy use of distorted electric guitars, bass guitar and drums. The term "hard rock" is often used as an umbrella term for genres such as punk, grunge and heavy metal in order to distinguish them from the more radio-friendly pop rock genre.
Hard rock is strongly influenced by blues music; the most frequently used scale in hard rock is the pentatonic, which is a typical blues scale. Unlike traditional rock and roll (which takes elements of the "old" blues), hard rock incorporates elements of "British blues", a style of blues played with more modern instruments such as electric guitars, drums, keyboards and electric bass. A notable departure from traditional blues forms is that hard rock is seldom restricted to the I, IV, and V chords prevalent in 12- or 16-bar blues, but includes other chords, typically major chords rooted on tones of the minor scale.
The term "hard rock" is often applied to many styles of rock music, their only common feature being that they deviate from pop rock, though this is generally incorrect. Two such examples are punk rock and grunge. Punk rock uses a faster tempo and fewer riffs (often using power chords). (Wilkipedia)