Many people confuse spark arrestors with chimney caps because they both sit on top of the chimney. However, not all chimney caps have the mesh metal siding that spark arrestors have.
A spark arrestor prevents the emission of flammable debris from sources of combustion. They are designed to keep sparks and embers from jumping out of the chimney. Spark arrestors usually have a double layer of metal mesh that catches embers but allows smoke and gases to exit the chimney.
Spark arrestors also have other functions. They keep animals, leaves, and other debris from entering the chimney. Without this protection, animals can nest in your chimney and cause clogging.