What is the difference between toffee and caramel?
In short, Butter. Toffee has Butter, caramel does not.
Of course there are lots of variations,  and there are some candies called 'Caramels', which may in fact be hard toffees.  The softness or hardness of a toffee depends on the amount of fat added, and the temperature to which the sugar is raised.

Strictly speaking, though, caramel is either 'dry' (pure white sugar), or 'wet' (white sugar and water), heated until it is browns, more or less, to taste, and stopped before burning.