The exact temperature depends on the concentration of salt and the type of salt used. As the human adds ice, the temperature drops below 32F, but the salt water solution doesn't freeze. When rock salt is added to the street, it depresses the freezing point of any water which dissolves it. This salt water solution can exist as a liquid at lower temperatures than fresh water. Salty water, instead of clean ice, if the solution is strong enough to withstand the surface temperature.
Sodium chloride is used because it is cheap and easy to obtain in large quantities.
But, as any New Yorker with a car can tell you, salt can be quite corrosive. And as hard as it is on cars, it's just as hard on roadways and bridge decks. This is costly in the long run. So, alternative methods to road salting are desirable. One type of alternative is using a different kind of salt.