How does salting roads help prevent ice?
Sodium chloride rock salt, NaCl, is usually used for de-icing roads. As mentioned in another answer, the dissolved ions lower the freezing point of water.

Other kinds of salt are also used. Calcium chloride, CaCl2, is more efficient at melting ice. NaCl can not melt ice at temperatures lower than -18C (0 F). A saturated calcium chloride solution won't freeze until ~ -46 C. Calcium chloride is often applied as a solution because the solid becomes sticky as it absorbs water from the air.

Edit: per the link to the Colorado road dept from a comment on another answer, magnesium chloride is used in the liquid deicers. Here is a link to information on different deicers