Is there any difference between fleur de sel and salt? I notice some of Martha Stewart's recipe have this?

Fleur de sel is a special sea salt from France. It comes from Brittany and has a grayish color because of the minerals in it and tends to look wet. It has a very delicate flavor and is usually used to finish off foods - you wouldn't put a tablespoon in your soup before boiling, but you would sprinkle a few grains over say a steak just before you eat it. It is also fairly expensive. Caramels with fleur de sel are actually a popular high end treat - her cake is probably intending to evoke that flavor. If you don't have the fleur de sel, don't use another regular salt to try to get the same flavor.