What is "Progressive Rock"?
Framed by its predecessor, the Psychedelic movement of the mid sixties, and by its so-called executioner, the Punk (or later New Wave) movement of the late seventies, Progressive Rock is firmly rooted in the analog era of the seventies. Thus, it is easiest to define Progressive Rock in the context of time and place; for our purpose, we will primarily look to the United Kingdom and Europe, circa 1967-1980. Outside of that, we have found three elements in particular that can be found in all Progressive Rock: 1) Foremost is virtuosity in execution. 2) Second is extended composition onto the basic structure(s) of rock music. 3) The final element is that it must engage the listener.

Basicly, if you're a fan of The Mars Volta, you might be a prog fan. Even if you've never heard any of the good stuff such as Yes, Rush, Pink Floyd, The Moody Blues, Deep Purple and King Crimson , ect. It's a dieing form of music. Save this shit because it's good. =D

Timeline : http://www.progressiverock.com/