As a big classic rock fan, I'll try to help you there.
Classic rock is like the proverbial definition of obscenity; you can't define it, but you know it when you hear it. Classic rock transcends dates and eras. You can have classic rock that predates the Beatles (Green Onions and anything by Chuck Berry) and you can have classic rock that was released last week (Drive-By Truckers' new record). The heart of the classic rock genre, though, runs from about 1966 through 1995.
It's generally guitar-driven, with a fairly standard format song-wise; that is, 2-3 verses, a guitar solo for the bridge, then a closing verse and maybe a closing solo. But, again, there is enormous variance in the song structure of a typical classic rock song. A classic rock song doesn't necessarily have to be electric either. There are plenty of classic rock songs that are totally acoustic (LZ's Going to California and Traffic's John Barleycorn being two examples off the top of my head). There is jazz-oriented classic rock (much of Steely Dan's repertoire) and country-flavored classic rock (much of Southern rock), and the blues are a major foundation for what we now consider classic rock.
I'm sure the music lovers on the board may have their own view of what is and isn't classic rock, but that's how I see it.