Oats are the whole grains or kernels that are the seed of the oat plant, while oatmeal commonly refers to the cereal or porridge made by cooking processed oats in a hot liquid. Coarsely ground oat kernels are also called oatmeal. Finely grinding oats creates oat flour.
There are several different varieties of oats and oatmeal that are available commercially. Rolled oats are the most common. Old-fashioned rolled oats are oat kernels that have been steamed until soft, rolled flat and then dried. Quick oats are steamed longer and rolled thinner than old-fashioned rolled oats. Instant oatmeal is made with rolled oats that are steamed and rolled extra-thin to cook nearly instantly. All varieties of rolled oats cook easily in boiling water to make oatmeal breakfast cereal.
Steel-cut oats are oat kernels that are sliced into a few thick pieces with a steel blade. They make oatmeal cereal with a coarser texture than rolled oats. Scottish oatmeal refers to oats that have been coarsely ground using milling stones. Scottish oatmeal makes an oatmeal cereal that is creamier than oatmeal made with rolled or steel cut oats.
Whole oat grouts are the minimally processed oat kernels. Oat grouts are a hearty grain similar to bulgar or barley and are typically added to soups, stews and casseroles.