Progressive rock (or prog rock )is a new form of rock that appeared in late 1960’s,mostly in England , combining elements from jazz, avant-garde , and classical music in contrast to American rock, which was a combination of blues and country music .
Over the next years various sub-genres of progressive rock have developed , such as: art rock,symphonic rock and progressive metal.
What was different about this gen of rock was that lyrics of songs are sometimes abstract or based in fantasy,covering themes as history, religion,science fiction, madness ,love ,war , social issus or even left-wing political ,then the compositions were longer , sometimes running over 20 minutes (for instant Jethro Tull’s “Thick as a Brick” takes 43 minutes to play ), prominent use of electronic instrumentation ,especially the organ, electric guitar, piano and drums.
Progressive rock has served as a inspiration for other genres such as : neo-classical and symphonic metal ,post-rock, avant-garde metal .

Among most successfull prog rock groups :

Yes
Pink Floyd
Porcupine Tree
Dream Theater
Led Zepellin
Jethro Tull
A perfect circle
Genesis
The flower kings
Magic pie
Little tragedies
Spock’s beard
Tristan park
Family
Gentle giant