Classic Prog (or Symphonic Rock) - This is the category where we find most of what we have come to know as progressive rock bands. The aspect of orchestration is the most important characteristic. Songs will be longish, contain extended solos for emphasis, changes in time and tempo, and more than any other category highlighted by strong dynamics and changes in mood. Very much an album oriented classification, like classical rock much was borrowed in terms of arrangement and structure. Lyrics involve many aspects, but typically more philosophical or fantasy oriented even in their telling of modern day subjects.
Canterbury Scene - It was from this area of England that a whole sub-genre of progressive rock sprang and the music created now bears its name. Typically this music is more intricate or complex, yet not without a sense of humor. A busy almost baroque style often wrapped in playing that resembles light jazz.
Art Rock - What perhaps typifies this music best is its need to be 'nice' with lots of major chords and bright harmonies. Very structured and even adventurous, sometimes heavy, sometimes mellow, strong melodies, good hooks are an integral part of most of the material. Bands in this category can and have produced material falling into other categories as well. There may be moments of outright progressive rock but with more of a pop influence and certainly a tendency towards shorter songs.
Space Rock - This is a term that goes back to the early days of Pink Floyd. Space Rock basically came to represent bands that used either guitar, organ or synthesizer breaks that extended themselves for extended periods using few tone changes. In some ways this might be called minimalist but only for certain segments of the songs in question. Usually long passages where little is happening other than mood or atmosphere being created. The need for lyrics is almost secondary, and if they existed at all it is usually for the purpose of setting up the narrative. Lyrics may revolve around some cosmic or science-fiction themes. I have intentionally included Electronic artists in this category.
Classical Prog (or Classical Rock) - The need for experimentation began to draw on many influences. One of the greatest sources had to be the classical composers. Contemporary rock bands might lift titles, bridges, bars, passages, in other words virtually everything in their quest to find the 'new'. No place was this more in evidence than in Italy. The material will also be very influenced by the classical style of arrangement and structure. The music tends to be composed of longer passages, containing dynamics, changes in time and tempo, lyrics usually referred to everything from romance to fantasy.
Neo-Prog - This style of progressive rock came into existence more or less in the early eighties and is generally based on a symphonic rock style, although with more of an eighties or nineties edge. The music is still in the long and short format but typically is based on an adventurous song style with more constant drumming. Lyrically neo-prog still dealt in philosophical matters but on a more down-to-earth, day-to-day level.
Pomp Prog - Possibly an offshoot of the whole progressive rock idea. This music is characterized by two basic qualities... 1) Numerous crescendos, possible changes in time and tempo, accompanied by relatively bombastic intros and extros, 2) The music tends to be relatively fast paced and heavier throughout. LP's tend to sound at first basically like hard rock with a few ballads thrown in. Lyrics tend to be on the aggressive side. This sub-genre led directly to prog metal bands like Dream Theater.
RIO (Rock-In-Opposition) - Trying to define a sub-genre of progressive rock that defies description may seem like a dead end. This style, originally very politicized is one originated by ex-Henry Cow drummer Chris Cutler. The music is extremely challenging and can be difficult to listen to. It's an acquired taste. Bands like Art Zoyd and Univers Zero generally fall into this category.
Zeuhl - A sub-genre of progressive rock created by the French band Magma that is really a unique and very challenging form of jazz fusion incorporating chanting, dissonance and loud rock. Stylistically there are elements of Bartok, Stockhausen, Duke Ellington, and blues shouting.