What is snow salt made of?

Depends on how cheap the city is. sodium chloride (table salt) is sometimes used because it is cheap, but it causes horrible rust. Calcium chloride is often used for it's lower melting point, and reduced corrosion, but it is more expensive. Ordinary sand is actually commonly used as well. It doesn't melt the snow, but it improves traction without causing slippery ice to form do to refreezing.