Just to take that excellent answer another step -- steel cut oats are hulled and chopped in pieces -- by a steel blade, no doubt.

The difference is mostly textural. Steel cut oats are denser, chewier and take a long time to cook (45 min.) Rolled oats are steamed and pressed into quick-cooking flakes with a familiar (to Americans) texture. Since they don't require further cooking, they present additional culinary uses.

Because steel cut oats are processed less, some people believe they're more healthful but that's not true. They're both whole grains, with no significant nutritional difference.