Progressive rock is a genre of rock music that emerged in Britain in the late '60s and had a brief heyday in the early '70s before fading from the mainstream consciousness almost immediately with the rise of punk rock, leaving an unseen but dedicated underground. It's called "progressive" because, especially initially, its primary focus was to make "progress" for rock and roll -- to push it to the limits of artistic merit and musical depth. You can see how punk is diametrically opposed to it.
Being as much of a philosophy about music as it is a genre of music, its influence are quite eclectic, ranging from folk music to classical music to jazz fusion. However, it does have a certain distinguishable style, being characterized by long, complex songs with long, complex solos, and many different musical themes throughout both -- key changes and even time changes are commonplace. A 5-minute track is considered short. The albums are rarely composed of songs that have no connection to one another; concept albums is considered the rule and not the exception.
Here are some bands that are considered to be a part of, or at least connected to the progressive rock movement:
- King Crimson, considered to be a major progenitor
- The Moody Blues, also quite influential
- later Pink Floyd -- Atom Heart Mother, The Dark Side of the Moon, The Wall, et al.
- Yes
- Emerson, Lake and Palmer. RIP Keith Emerson.
- Jethro Tull
- earlier Genesis -- Foxtrot, Selling England by the Pound, The Lamb Lies Down on Broadway et al.
- earlier Styx -- Man of Miracles, Crystal Ball, The Grand Illusion, et al.
- Kansas