I like prog rock too, I'd reckon those guys just start the recorder on start jamming cause as far as chord structures concerned it all over the place, compared to "normal" songs prog rock doesn't repeat (e.g verse chorus verse chorus). Which is great to hear if done properly cause the music has various moods, slow then fast then slow etc, quiet then loud.. alternates frequently.
Well I don't know the textbook definition but that's my interpretation :)
Btw where can you get definitions of styles? e.g. alternative (wtf is that?), grunge (wtf is that x2), hmm what else is there? What would you call Yngwie's style? Classical rock or Baroque (is that how you spell it?). Give me some example :)