The terms "popular music" and "pop music" are often used interchangeably, although the former is a description of music which is popular (and can include any style). So-called "pure pop" music, such as power pop, which has been described as mixture of hard rock and melodic pop music and tends to be more aggressive than pop rock, features all these elements, using electric guitars, drums and bass for instrumentation. The sound of rock is traditionally centered on the 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 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 others, 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 chord instrument, bass, and drums). A group of musicians performing rock music is termed a rock band or rock group and typically consists of between two and five members. So-called "pure pop" music, such as power pop, which has been described as mixture of hard rock and melodic pop music and tends to be more aggressive than pop rock, features all these elements, using electric guitars, drums and bass for instrumentation. 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.
Like pop music, lyrics often stress romantic love but also address a wide variety of other themes that are frequently social or political in emphasis. 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. From about 1967 the term was increasingly used in opposition to the term rock music, a division that gave generic significance to both terms. Whereas rock aspired to authenticity and an expansion of the possibilities of popular music, pop was more commercial, ephemeral and accessible. According to Simon Frith pop music is produced "as a matter of enterprise not art", is "designed to appeal to everyone" and "doesn't come from any particular place or mark off any particular taste".
The term "pop song" is first recorded as being used in 1926, in the sense of a piece of music "having popular appeal". Hatch and Millward indicate that many events in the history of recording in the 1920s can be seen as the birth of the modern pop music industry, including in country, blues and hillbilly music. From the 1990s alternative rock began to dominate rock music and break through into the mainstream in the form of grunge, Britpop, and indie rock. Like pop music, lyrics often stress romantic love but also address a wide variety of other themes that are frequently social or political in emphasis. So-called "pure pop" music, such as power pop, which has been described as mixture of hard rock and melodic pop music and tends to be more aggressive than pop rock, features all these elements, using electric guitars, drums and bass for instrumentation. Rock music has also embodied and served as the vehicle for cultural and social movements, leading to major sub-cultures including mods and rockers in the UK and the hippie counterculture that spread out from San Francisco in the US in the 1960s. 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. records for singles "revolutionized the manner in which pop has been disseminated" and helped to move pop music to 'a record/radio/film star system'. Another technological change was the widespread availability of television in the 1950s; with televised performances, "pop stars had to have a visual presence". In the 1960s, the introduction of inexpensive, portable transistor radios meant that teenagers could listen to music outside of the home. Multi-track recording (from the 1960s); and digital sampling (from the 1980s) have also been utilized as methods for the creation and elaboration of pop music. By the early 1980s, the promotion of pop music had been greatly affected by the rise of Music Television channels like MTV, which "favoured those artists such as Michael Jackson and Madonna who had a strong visual appeal". Pop music (a term that originally derives from an abbreviation of "popular") is a genre of popular music, which originated in its modern form in the Western world during the 1950s and 1960s, deriving from rock and roll. Pop music has been dominated by the American and (from the mid-1960s) British music industries, whose influence has made pop music something of an international monoculture, but most regions and countries have their own form of pop music, sometimes producing local versions of wider trends, and lending them local characteristics. Some of these trends (for example Europop) have had a significant impact of the development of the genre.