Cornmeal is ground up corn. It is not to be confused with cornstarch or corn masa. Both cornstarch and finely-ground cornmeal are sometimes called cornflour; this is thus a bad term to use.
Cornmeal is commonly used to keep baked goods from sticking, though it is not used on sweets. It would be used for pizza or bread.
Cornmeal can be applied to wet items, drying them off so that batter will stick. This is commonly done for deep-fat frying. When a recipe uses cornmeal in this way, you can often substitute flour or cornstarch.
Cornmeal is commonly used in batter for deep-fat frying.