Adding salt raises boiling point and lowers freezing point. These are two of the four colligative properties of aqueous solutions.
The salt disassociating on the ice causes the water on the ice to have a lower freezing point. There is also an exothermic reaction (enthalpy) involved with some salts during disassociation and this can cause the ice to melt and continue the reaction. Magnesium chloride (that's the stuff CDoT sprays on the roads here) is one type of salt that does this.
I'm currently going to Metro for a BS in Chemistry, Criminalistics concentration. I average about 40 hours a week in the science building, all of that in the chemistry wing. My Gen Chem teacher calls me a 'chemistry whore'. The more I learn the more I want. I don't always get everything right, but I'm still learning.