The gizzard is an organ found in the digestive tract of some animals, including archosaurs (dinosaurs, birds, pterosaurs, crocodiles and alligators), earthworms, some gastropods, some fish and some crustaceans. This specialized stomach constructed of thick, muscular walls is used for grinding up food, often aided by particles of stone or grit. Some animals that lack teeth swallow the stones or grit for this purpose. The small bits of gravel that act as 'teeth' in the gizzard, break down hard food such as seeds and thus help digestion. All birds have gizzards, but not all swallow stones or grit.

There are two kinds of poultry grit.
- Insoluble - Commonly crushed granite. It is used in the gizzard.
- Soluble - Commonly ground oyster shell. It dissolves and provides calcium to make egg shells harder.

Both can be sprinkled on ice to increase traction.
Ground oyster shell can also be sprinkled around bulb plantings to keep animals from digging them up.

Links:
Grit and Oyster Shell; When in Doubt, Put it Out | Backyard Poultry Magazine