Freezing and Melting
Ice, and by association the water that is around ice, isn't as static or as simplistic as it may first appear. When the temperature of water is at the freezing point--0 degrees Celsius for the following examples--ice is actually in fluid motion. What may look like a solid sheet of ice over water is actually a constantly shifting thing, with water freezing at the exact same rate that ice is melting. As long as these two rates--the freezing and melting rates--stay the same, you won't notice the change that's taking place. However, if something is added to the water and ice solution, the addition will upset this delicate balance. This is particularly true if you add to the water something like rock salt, which changes the balance entirely.
Adding rock salt to a slurry of ice and water lowers the temperature at which water freezes. If you added rock salt to a glass of ice and water, the ice would melt more slowly, and the general temperature of the water would go down. The reasons for this are somewhat complicated, but this simplification gives some of the pertinent details.
Basically, for ice to melt, energy must be drawn from the surrounding solution to break the hydrogen bonds that keep the ice frozen. The energy that's taken is in the form of heat, which is why ice makes the solution that it's in cold, since it's taking the heat to melt. Salt upsets the balance and makes the melting rate slower, because the ice requires more energy to melt. This draws more heat from the solution, which results in a larger temperature drop.
The knowledge of how rock salt can interact with water and ice to form temperature differences led to one of the most common summertime treats available--namely, ice cream. Old-fashioned ice cream makers used rock salt to lower the temperature around the buckets that the ice cream was being made in. This was done by packing a mixture of water, ice and rock salt all around the bucket that contained the ice cream mix, thereby lowering the temperature. Using this formula, the early ice cream makers could lower the temperature to an effective -21 degrees Celsius during the churning process and keep the temperature there to make sure their product didn't melt.