"After the outer husk (or chaff) has been removed from the still bran-covered oat grains, the remainder is called oat groats. Oat groats are a whole grain that can be used as a breakfast cereal. Steel-cut oats are oat groats that have been chopped into smaller pieces and retain bits of the bran layer. Since the bran layer, though nutritious, makes the grains tough to chew and contains an enzyme that can cause the oats to go rancid, raw oat groats are often further steam-treated to soften them for a quicker cooking time (modern "quick oats") and to denature the enzymes for a longer shelf life.
Rolled oats that are sold as oatmeal usually, but not always, have had the tough bran removed. They have often, but not always, been lightly baked or pressure-cooked or "processed" in some fashion. Thick-rolled oats are large whole flakes, and thin-rolled oats are smaller, fragmented flakes. Oat flakes that are simply rolled whole oats without further processing can be cooked and eaten as "old-fashioned" oatmeal, but more highly fragmented and processed rolled oats absorb water much more easily and therefore cook faster, so they are sometimes called "quick" or "instant" oatmeal."
"When bran is removed from grains, the latter lose a portion of their nutritional value...Bran is particularly rich in dietary fiber and omegas and contains significant quantities of starch, protein, vitamins, and dietary minerals."