To me, 'rock' is a very loose umbrella term for all kinds of largely electric-guitar-based music that can be traced, in one way or another, back to the original rock'n'roll of the 50s. So this would include punk, metal, indie and pop-rock, for example.
Rock'n'roll has a much more specific sound to it; generally it's either rock of the style played in the 50s, with very clear blues, country and rockabilly influences, or in some sub-genre more directly descended from it (as in the case of the Misfit's 'psychobilly', as demonrail points out, despite the fact they started a good 20 years and more after the first wave of rock'n'roll).
I think the difference is summed up quite nicely by the first two songs on the fourth Led Zep album: you've got Black Dog, which is most definitely rock, followed by Rock And Roll, which is exactly that (and, having been recorded in 1971, was already knowingly anachronistic by that time).