Metal is a form of rock, not the other way around, and because of that sometimes the lines can get blurry depending on the artist. Both Rock and Metal have quite a few different sub-genres. One of the main differences of rock and metal in the "modern" sense (well after metal split from rock and became more distinguishable), is rock usually has more of a back-beat and the bass guitar is usually more prominent. Another thing, is rock usually (but not always), has stronger ties to its blues-roots, where metal is usually (but not always) a bit strayed away from its blues roots, having a more "metallic" feel.
But, the lines can get blurry with alot of modern hard rock bands that have some heavy metal elements. For example, Disturbed is mostly hard rock, but they seem to have infused quite a bit of heavy metal elements, I think even the lead singer mentioned they were a rock/metal hybrid.