The most simple and straightforward definition of art rock is that it's rock 'n' roll made by art-school kids. This is most certainly the case with the genre's flagship band, the great Roxy Music. Both the band's founder, Bryan Ferry, and synthesizer weirdo Brian Eno were art brats who wanted to filter the ecstatic awesomeness of rock through avant-garde concepts and tactics unique to modern painting and sculpture. Consequently, art rock can be seen as something of the bridge between progressive rock (pretentious and complex) and glam (joyously tacky and ostentatious). This was especially true in the genre's 1970s heyday, with such artists as David Bowie, Lou Reed, Robert Wyatt and Peter Gabriel, all of whom vacillated between these two poles.
But there's another, somewhat more subversive, strand of art rock, one that emerged in the wake of The Beatles' 1969 psych-masterpiece Sgt. Pepper's Lonely Hearts Club Band. This, too, thrived in the '70s, centering around mighty Electric Light Orchestra, Roy Wood, Sparks and 10cc. These cats were far more into radio-pop perfection than were Ferry, Bowie, Eno, et al. After all, none of them ever made a soft-rock tune as sublime as "I'm Not In Love"! This primer will get you started.