Oats are a cereal grain (Avena Gramineae, to be specific) domesticated since prehistoric times. Though the ancient Greeks and Romans apparently viewed this grass as a weed, oats were a staple food source in Scotland and other parts of Europe until the 19th century. Oats are a linguistic oddity: they are the only grain commonly referred to in English in a collective plural. (We don't say rices or wheats, do we?)
Oats have a pleasant nutty flavour and are rich in soluble fibre, giving them cholesterol-fighting properties. Because they maintain their bran and germ during processing, they are more nutritious than many other processed grains.
Many adults loathe oats because of their association with that dreaded childhood food, oatmeal porridge; I too was forced to eat gloppy grey porridge as a child, the horror only mitigated by the copious amounts of brown sugar I was allowed to sprinkle on top. But oats are a versatile and nutritious food that, now we're adults, we can learn to love again. Or something like that.
Oats grow best in cool, moist climates, but can adapt to very poor soils where other grains cannot grow. Whole oats are long thin grains resembling millet which are used as animal fodder; humans don't generally consume oats until they have been cleaned, toasted, and hulled, at which time they are known as groats. Whole oat groats can be cooked like rice and served as a side dish. Steel-cut oats or scotch oats are groats that have been cut into two or three pieces; they have a nice chewy texture and are delicious toasted in butter before being boiled, but they take quite a long time to cook.
More commonly, though, the groats are flattened by a process which involves steaming and rolling them between huge rollers; this produces flat flakes which are often called old-fashioned or rolled oats. These can be cooked in water - one part oats to two or three parts liquid, brought to the boil and then simmered for 10 to 20 minutes. This is the grey glop of my childhood. Quick-cooking oats are just rolled oats that are cut thinner to shorten cooking time; they'll achieve glop consistency in a mere three to five minutes. Instant oats are partially cooked and then dried and rolled thin; this is the stuff that comes in little packages in the breakfast cereal section, the flavour "enhanced" with the addition of lots of sugar and salt and chemical flavour enhancers. It cooks just by pouring boiling water over.
Oat bran may be present in steel-cut or rolled oats, but can also be bought as a separate product; it's higher in fibre than wheat bran. Oat flour contains no gluten and so isn't often used to make bread; it is sometimes used in addition to wheat flour in baked goods, producing a dense product.
The Visual Food Encyclopedia