What are the charactoristics of progressive rock?

Progressive rock ("prog") is a somewhat subjective term but I'll do my best to answer. Put simply, progressive rock is a movement that began in the late 1960s, as rock bands began to experiment with their sound more and more. While there is no set "litmus test" for what is and isn't prog, there are certain characteristcs that most prog bands embody. First, they are always looking for a way to push the boundaries of the genre in some way. This could mean experimenting with theatrics (Genesis), incorporating an orchestral sound (The Moody Blues), fusing classical and jazz idioms with the rock sound (Emerson, Lake & Palmer), or adapting a largely improvisational approach (King Crimson). Most, if not all, of these bands also pushed the bounds of rock musicianship, showcasing the members with virtuosic passages and solos. Additionally, most prog can be identified by the approaches used in the songwriting itself. Most pop/Rock Music will follow a verse/chorus/bridge format, not deviating frequently if at all. Prog rock, on the other hand, can be the fusion of very distinct song sections that may come back (or not!). It's also not uncommon for a prog rock track to exceed the 3-4 minute length most Pop Music has. For instance, the band Yes released three albums from 1972-1974, the first and last of which contained one side-long track (~20 minutes) and two tracks around 9-10 minutes. And in the middle was Tales From Topographic Oceans, a double album containing four tracks, each between 18-21 minutes in length and covering an incredible array of musical styles.

There are also bands on the fringes of the genre that some will claim belong, and others will say they do not belong. For instance, Styx, Queen and Uriah Heep all have songs/albums that fit the above definition of prog, but then also have many pop-oriented albums thatlack the ambition of the bands listed above. Ultimately, it's up to the listener to decide if they're hearing prog.