Take corn (sweetcorn or maize) kernels, dry them, and grind them, and that's cornmeal. It's usually yellow, but can be white or blue, depending on the type of corn used.
There are two methods of grinding the corn: an old-fashioned way using a water-powered stone mill, and a new-fangled way using steel rollers. The result of the former is known as stone-ground or water-ground, and it's preferable because it retains the hull and germ of the corn, so the end result is more nutritious and tasty. The downside is it's also more perishable, and must be kept in the fridge. The steel rollers rid the corn kernels of their husk and germ - the perishable bits - and so the meal can be kept almost indefinitely. If the package doesn't tell you how it's ground, it's probably steel-ground; only stone-ground is usually specifically labelled.
Because cornmeal is often used to make the Italian staple polenta, the meal itself is sometimes called polenta. Cornmeal is also used in baked goods, particularly breads and muffins. It's good for dusting a pizza peel when baking homemade pizza.
Cornmeal can be milled coarse, medium, or fine, and in the latter form is known in North America as cornflour, which is kind of confusing, because in Britain and probably other places as well, cornflour is used to refer to what North Americans call cornstarch, made from finely ground endosperm of corn.
Corn kernels can also be processed in lye or lime water to remove the hull and germ, then ground; this meal is known as hominy or grits. Masa or masa harina, finely ground hominy, is used to make tortillas.