By the late 1960s "classic rock" period, a number of distinct rock music subgenres had emerged, including hybrids like blues rock, folk rock, country rock, raga rock, and jazz-rock, many of which contributed to the development of psychedelic rock, which was influenced by the countercultural psychedelic and hippie scene. New genres that emerged included progressive rock, which extended the artistic elements; glam rock, which highlighted showmanship and visual style; and the diverse and enduring subgenre of heavy metal, which emphasized volume, power, and speed. In the second half of the 1970s, punk rock reacted by producing stripped-down, energetic social and political critiques. Punk was an influence in the 1980s on new wave, post-punk and eventually alternative rock. From the 1990s alternative rock began to dominate rock music and break into the mainstream in the form of grunge, Britpop, and indie rock. Further fusion subgenres have since emerged, including pop punk, electronic rock, rap rock, and rap metal, as well as conscious attempts to revisit rock's history, including the garage rock/post-punk and synth-pop revivals at the beginning of the 2000s.
Rock music has also embodied and served as the vehicle for cultural and social movements, leading to major subcultures including mods and rockers in the UK and the hippie counterculture that spread out from San Francisco in the US in the 1960s. Similarly, 1970s punk culture spawned the goth, punk, and emo subcultures. Inheriting the folk tradition of the protest song, rock music has been associated with political activism as well as changes in social attitudes to race, sex and drug use, and is often seen as an expression of youth revolt against adult consumerism and conformity.
The sound of rock is traditionally centered on the amplified electric guitar, which emerged in its modern form in the 1950s with the popularization of rock and roll, and was influenced by the sounds of electric blues guitarists. The sound of an electric guitar in rock music is typically supported by an electric bass guitar, which pioneered in jazz music in the same era, and percussion produced from a drum kit that combines drums and cymbals. This trio of instruments has often been complemented by the inclusion of other instruments, particularly keyboards such as the piano, Hammond organ and synthesizers. The basic rock instrumentation was adapted from the basic blues band instrumentation (prominent lead guitar, second chordal instrument, bass, and drums). A group of musicians performing rock music is termed a rock band or rock group and typically consists of between three–the power trio–and five members. Classically, a rock band takes the form of a quartet whose members cover one or more roles, including vocalist, lead guitarist, rhythm guitarist, bass guitarist, drummer and often that of keyboard player or other instrumentalist.
Rock music is traditionally built on a foundation of simple unsyncopated rhythms in a 4/4 meter, with a repetitive snare drum back beat on beats two and four. Melodies are often derived from older musical modes, including the Dorian and Mixolydian, as well as major and minor modes. Harmonies range from the common triad to parallel fourths and fifths and dissonant harmonic progressions. Rock songs, since the late 1950s and particularly from the mid-1960s onwards, often used the verse-chorus structure derived from blues and folk music, but there has been considerable variation from this model. Critics have stressed the eclecticism and stylistic diversity of rock. Because of its complex history and tendency to borrow from other musical and cultural forms, it has been argued that "it is impossible to bind rock music to a rigidly delineated musical definition."
Unlike many earlier styles of popular music, rock lyrics have dealt with a wide range of themes in addition to romantic love: including sex, rebellion against "The Establishment", social concerns and life styles. These themes were inherited from a variety of sources, including the Tin Pan Alley pop tradition, folk music and rhythm and blues. Music journalist Robert Christgau characterizes rock lyrics as a "cool medium" with simple diction and repeated refrains, and asserts that rock's primary "function" "pertains to music, or, more generally, noise." The predominance of white, male and often middle class musicians in rock music has often been noted and rock has been seen as an appropriation of black musical forms for a young, white and largely male audience. As a result, it has been seen as articulating the concerns of this group in both style and lyrics. Christgau, writing in 1972, said in spite of some exceptions, "rock and roll usually implies an identification of male sexuality and aggression".
Since the term rock began to be used in preference to rock and roll from the late-1960s, it has often been contrasted with pop music, with which it has shared many characteristics, but from which it is often distanced by an emphasis on musicianship, live performance and a focus on serious and progressive themes as part of an ideology of authenticity that is frequently combined with an awareness of the genre's history and development. According to Simon Frith "rock was something more than pop, something more than rock and roll. Rock musicians combined an emphasis on skill and technique with the romantic concept of art as artistic expression, original and sincere". In the new millennium the term rock has sometimes been used as a blanket term including forms such as pop music, reggae music, soul music, and even hip hop, with which it has been influenced but often contrasted through much of its history.
The foundations of rock music are in rock and roll, which originated in the United States during the late 1940s and early 1950s, and quickly spread to much of the rest of the world. Its immediate origins lay in a melding of various black musical genres of the time, including rhythm and blues and gospel music, with country and western. In 1951, Cleveland, Ohio disc jockey Alan Freed began playing rhythm and blues music (then termed "race music") for a multi-racial audience, and is credited with first using the phrase "rock and roll" to describe the music.