is a broad and convergent style of rock music and progressive music which arose in the late 1960s, reaching the peak of its popularity in the early 1970s, but continuing as a musical form to this day. Progressive rock is often closely identified with other styles of music such as experimental music, symphonic rock, art rock and progressive metal.
Progressive rock artists sought to move away from the limitations of the radio formated rock, mainly its cyclic structure, favoring a progressive one (hence the term "progressive"). Progressive rock is often wrongly percieved as complex and elaborate music, requiring a high level of musicianship from the artists, mainly because of the impression left by some of the most popular progressive bands. --http://en.wikipedia.org/wiki/Progressive_rock
Here are some popular progressive rock bands:
4. Pink Floyd
And many others....
While the period does not have an exact "starting date" many rock historians and musicians who played at that time believe that the Progressive Rock Movement was set in motion by the release of the Beatles Sgt. Pepper's Lonely Hearts Club Band in 1967.
While traditional rock and roll is ultimately based on the blues, progressive rock tends to be based more in European classical music and post bop jazz. Gustav Holtz's Mars was a concert staple at King Crimson concerts in the early 70's and Emerson, Lake and Palmer put Copeland's Hoedown and Mussorgsky's Pictures at an Exhibition in their repertoire. Because European classical music is known for grandiose instrumental passages and post bop jazz is known for improvisation, the influence they provided to the structure of progressive rock cause a division on how the movement was, and is, viewed in rock history.