Nancy and I sat here recently and watched all the Dream Theater vids on YouTube. Exceptional musicians, all; tighter than a frog's ass.
A whole lot of what we simply called "rock" back in the early 70's would probably be called prog rock today, what with the likes of early Genesis, Pink Floyd, ELP, King Crimson, Hawkwind, Touch, Steamhammer, Armageddon, Jethro Tull, Kansas, Yes, etc. Now they're old, I'm old, and everyone just calls it "classic rock". It set the stage for what was to come, though. Complex music that wasn't just three chords and a chorus.
Nowadays I don't know so much about it, though a couple years ago I bought SB, DT, and a couple Ozric Tentacles along with a couple others just for grins.