Any type of salt lowers the freezing point of water to -8'c. This makes snow/slush change state (from frozen to liquid) which requires a lot of energy to do... This energy is stolen from the snow around the salted area, causing it to drop in temperature, which makes it bond to the snow beneath (hardens the snow.) When the melted water drains, voila! You're left with a nice hard surface! Man, am I ever a nerd?!