Progressive Rock was a music genre that rose into popularity in the late 1960s and early to mid 1970s, and it has since became infamous for being perceived as overblown and pompous.
Whereas rock music traditionally drew from rhythm and blues, progressive rock expanded that sound by adding classical and jazz inspirations. The songs tended to be long, sometimes with multiple parts, similar to classical symphonies. A handful of musicians even played with several prominent bands at different points in their careers.
King Crimson endured many, many lineup changes over the years, with founder and guitarist Robert Fripp (far right on the image above) as the only constant. Fripp was known for using very unorthodox guitar tunings. The original 1969 lineup featured Greg Lake (of Emerson, Lake & Palmer fame) on vocals and bass, Michael Giles on drums, the lyrics of Peter Sinfield, and Ian McDonald (who later founded Foreigner) on various woodwinds and keyboards. Drummer Bill Bruford of Yes later joined the band in the 1970s.
From their debut album "In the Court of the Crimson King:"
From the second album (Greg Lake's last before joining Emerson, Lake & Palmer):
Emerson, Lake & Palmer
Keyboardist Keith Emerson, vocalist and bassist/guitarist Greg Lake, and drummer Carl Palmer each left popular groups to form their own super-group in 1970. Emerson was fascinated with classical music, and that had a heavy influence on their sound. He was also known for his stage antics, which included playing a rotating piano and attacking a Hammond organ.
Emerson helped to popularize the Moog Modular synthesizer with his solo at the end of "Lucky man."
Yes has had a very long and storied career. Their albums are known for long compositions, some of which filled up entire record sides. "Tales from Topographic Oceans" was a double album with only four songs, one for each record side. Their classic lineup included Jon Anderson on vocals, Rick Wakeman on keyboards, Bill Bruford on drums, Steve Howe on guitar, and Chris Squire on bass.
"Roundabout" is one of their more radio-friendly songs from the 1970s.
"Close to the Edge" is a personal favorite of mine.
If your experience of the band Genesis is the Phil Collins-as-frontman era, you might be surprised to know that their sound from the 1980s and onward is worlds apart from the original lineup from the late 1960s through the early 1970s. It featured Peter Gabriel on vocals, Phil Collins on drums, and Steve Hackett on guitar.
This song goes back and forth between light and heavy, with many different instruments.
Gabriel was incredibly theatrical onstage, as this live performance of "Supper's Ready" shows.