Classic Rock is kind of a tough category to pigeonhole. True, it's a brand of oldies rock, but fifties music doesn't qualify (too old) and, to purists at least, neither does most eighties music (too new). If you find this at all confusing, you are hardly alone.
Broadly defined, classic rock is music that was first popularized during the FM Rock era of the late sixties and seventies. The cut-off, if there is one, comes at roughly the same time that the MTV era began (although many also consider bands of the same period like U2, R.E.M., and Dire Straits as classic rock).
Mostly, though, classic rock is a radio format created by onetime FM Rock radio programmers who found themselves in a time warp when they didn't expand their playlists beyond groups like Led Zeppelin, the Beatles, and the Stones. Other classic staples include Boston, Bad Company, Bruce Springsteen, Lynyrd Skynyrd and the Who.
As a radio format, classic rock remains hugely viable and such stations as Seattle's KZOK regularly top the ratings in their markets. You'll find classic rock fans talking about their favorite music at blogs like Classic Rock Central, Layla's Classic Rock and Classic Rock Revisited.