The salt used on roads is often called rock salt, because its grains are much coarser than table salt. It's still the same molecule—sodium chloride—but table salt is ground, purified, and often has additives like iodine (in order to decrease the incidence of goiters) and anti-clumping agents.
Most rock salt for roads is mined "dry" from underground seams of crystal salt, which formed from the evaporation of ancient seas. Miners follow shafts underground and break out slabs of the salt with dynamite and powered shoveling machines. Trucks or conveyors haul the salt to crushing machines.