An ice pack (commonly used therapeutically) is a plastic sac of crushed or cubed ice, refrigerant gel or liquid, or even frozen vegetables.
This refrigerant, usually non-toxic, can absorb a lot of heat, since its specific heat is very high.
It is commonly used to alleviate the pain of minor injuries.
The most common type of ice pack is simply a sack, bag, or towel filled with cubed or crushed ice.
Ice packs are used in coolers to keep disease-bearing foods (meats, milk products, eggs, etc.) below to keep them safe for eating. If the foods and the ice packs fill the cooler directly from the freezer, then the equivalent of 10 to 20 pounds of ice is needed for each 24 hour period. If the ice pack is filled with a coolant, then the same weight may last longer. If the foods come from the refrigerator then they will not stay cool as long with the same size ice pack. These foods should remain over and under for no longer than 4 hours accumulated over their entire existence. In that way, ice packs can be considered equivalent to a larger mass of ice.
Ice packs have the added benefit over ice that they do not cross-contaminate foods as ice can do when it turns liquid and mixes with the foods.
Another type of ice pack uses the endothermic reaction of ammonium nitrate and water to cool down quickly. When one breaks a tube inside the pack the ammonium nitrate is released allowing it to mix with the water.