AC/DC is hard rock, Black Sabbath is metal.
Van Halen is hard rock, Iron Maiden is metal.
Guns N' Roses is hard rock, Metallica is metal.

The difference lies within the melody. Rock 'n' roll has more melody, still has the blues, the r'n'b in it. Metal is completly free of that.

As Joe Perry was saying it: "Metal is not sexy, hard rock is!"

Of course, a lot of the times you have borderline cases like early Judas Priest records, WASP or some Twisted Sister. Actually, I am sure nobody can give you a perfect definition (although Iceman will pretend he did). I always listen to my ear. Hard Rock is simply more FUN!