Critic John Rockwell says that art rock is one of rock's most wide-ranging and eclectic genres with its overt sense of creative detachment, classical music pretensions, and experimental, avant-garde proclivities. In the rock music of the 1970s, the "art" descriptor was generally understood to mean "aggressively avant-garde" or "pretentiously progressive". "Art rock" is often used synonymously with progressive rock. Historically, the term has been used to describe at least two related, but distinct, types of rock music. The first is progressive rock, while the second usage refers to groups who rejected psychedelia and the hippie counterculture in favor of a modernist, avant-garde approach defined by the Velvet Underground. Essayist Ellen Willis compared these two types:
From the early sixties … there was a counter-tradition in rock and roll that had much more in common with high art—in particular avant-garde art—than the ballyhooed art-rock synthesis [progressive rock]; it involved more or less consciously using the basic formal canons of rock and roll as material (much as pop artists used mass art in general) and refining, elaborating, playing off that material to produce … rockand-roll art. While art rock was implicitly based on the claim that rock and roll was or could be as worthy as more established art forms, rock-and-roll art came out of an obsessive commitment to the language of rock and roll and an equally obsessive disdain for those who rejected that language or wanted it watered down, made easier … the new wave has inherited the counter-tradition.
Art rock emphasizes Romantic and autonomous traditions, in distinction to the aesthetic of the everyday and the disposable embodied by art pop. Larry Starr and Christopher Waterman's American Popular Music defines art rock as a "form of rock music that blended elements of rock and European classical music", citing the English rock bands King Crimson, Emerson, Lake & Palmer, and Pink Floyd as examples. Common characteristics include album-oriented music divided into compositions rather than songs, with usually complicated and long instrumental sections, symphonic orchestration. Its music was traditionally used within the context of concept records, and its lyrical themes tended to be "imaginative" and politically oriented.
Differences have been identified between art rock and progressive rock, with art rock emphasizing avant-garde or experimental influences and "novel sonic structure", while progressive rock has been characterized as putting a greater emphasis on classically trained instrumental technique, literary content, and symphonic features. Compared to progressive rock, art rock is "more challenging, noisy and unconventional" and "less classically influenced", with more of an emphasis on avant-garde music. Similarities are that they both describe a mostly British attempt to elevate rock music to new levels of artistic credibility, and became the instrumental analog to concept albums and rock operas, which were typically more vocal oriented.
Art rock can also refer to either classically driven rock, or to a progressive rock-folk fusion. Bruce Eder's essay The Early History of Art-Rock/Prog Rock states that "'progressive rock,' also sometimes known as 'art rock,' or 'classical rock'" is music in which the "bands [are] playing suites, not songs; borrowing riffs from Bach, Beethoven, and Wagner instead of Chuck Berry and Bo Diddley; and using language closer to William Blake or T. S. Eliot than to Carl Perkins or Willie Dixon."