There are two types of hand warmers: disposable and reusable. The disposable type relies on a chemical reaction to release heat that warms your hands, while the reusable type uses a physical change to achieve the same effect.
Disposable hand warmers contain powdered iron, salt, water, activated carbon and an absorbent material. The pouch containing this mixture is permeable so air can diffuse through it. Reusable types, by contrast, contain sodium acetate and water.
When the outer covering of a disposable hand warmer is removed, air diffuses through the permeable pouch and reacts with the iron to form iron (III) oxide. This chemical reaction is very similar to the one that forms rust on your car, and releases enough heat to warm your hands. In a reusable hand warmer, on the other hand, the heat does not come from a chemical reaction but from a physical change, where the rapid crystallisation of sodium acetate from liquid to solid form releases energy as heat.
Boiling a reusable hand warmer will melt the sodium acetate; once the hand warmer is cooled, the sodium acetate will remain in liquid form until the metal disk in the hand warmer is clicked, which will trigger recrystallization. Both the physical change in a reusable warmer and the chemical reaction in a disposable warmer are examples of exothermic processes.