Progressive Rock, also sometimes called Prog Rock or Prog, is a musical subgenre that originated in the United Kingdom with further developments in various other European throughout the mid-1960's to the 1970's and developed mainly from Psychedelic Rock. Bands moved more from the short pop single in favor of instrumentation and compositional techniques that were found more in Jazz or Classical Music in an attempt to give Rock a level of musical sophistication and critical respect.
The genre seemed to be a very experimental one, with one noticeable characteristic being the tendency to incorporate symphonic musical themes, as well as having extended musical themes, mystical and/or surreal lyrics, philosophical and/or spiritual themes, and complex orchestrations.
The genre seemed to enjoy much success during the 1970's, with several groups achieving either moderate or great success such as Queen, Pink Floyd, Yes, Rush, The Moody Blues, Genesis, King Crimson, Emerson, Lake & Palmer (often abbreviated as ELP), and more. In fact there are still some contemporary bands that either are considered Progressive Rock, or incorporate Progressive Rock elements, such as Muse, My Chemical Romance's "The Black Parade", Dream Theater (Though most of their music is Progressive Metal), and more.
The Author was heavily influenced by Progressive Rock, and in fact their early draft of Spectral Shadows was one that would be a Rock Opera stage show with most, if not all, of the backing music consisting of Progressive Rock. There is also a huge number of Prog Rock in-jokes and references that cover a variety of elements, from character names (the final boss of Aslander is also known as The Crimson King), to locations (The planet Cygnus, taking its name from Rush's "Cygnus X-1"), to serial and episode titles (S11x55 "Miracles Out of Nowhere" takes its name from a Kansas song), organizations (a group of lawyers in Suburbia is known as "ELP"), even lines of dialogue can contain references to Progressive Rock (One such example is in S11x13 "The Furry Within", where the spiritual forms of Perry, Christine, and Vicki encounter Ra, who proceeds to play a record in reverse while proclaiming "The music is reversible but time is not! Turn back!" in reference to Electrical Light Orchestra's Fire on High). There are an innumerable amount of these references and occasionally fans will discover references that even the author forgot about.
For more information, see the Wikipedia article on Progressive Rock.