If you've ever eaten Oatmeal cookies, instant Oatmeal or old fashioned Oatmeal, chances are you've had rolled oats. They are the most popular due in part to their fast cooking time.
Oats are processed into rolled oats by first removing the outer hull or husk, yielding “groats.” The groats are then cleaned, steamed, toasted, steamed again to soften them, and finally, rolled. Rolled oats that are not cut into smaller pieces are called “old fashioned.” They take about 5 minutes to cook on the stove, and they yield a chewier product after cooking.
Groats cut into a couple of pieces before being steamed and rolled are called “quick oats,” which cook on the stove in about a minute. “Instant” Oatmeal is steamed yet again and flavors, sweeteners and other ingredients may be added.
Steel cut oats
Ever since a few friends of mine brought home their first canister of these oats, They've sworn by them. They love the chewy crunch and the after-effect they have on the digestive system. Really, without getting graphic, Steel Cut oats drag everything out.
According to the Cook's Thesaurus, "These are groats that have been chopped into small pieces. They’re chewier than rolled oats, and grain aficionados often prefer them for hot Oatmeal cereals and muesli."