Specifically, steel cut oats look like little pellets, whereas rolled oats have the familiar flat shape from going through a steel roller. The rolled ones expose more surface area and therefore cook faster, but produce a different final texture. I do like the rolled ones better, but they take upwards of half an hour to cook. You can make a large batch and reheat it for 2-3 days if you like. I think some folks also use a pressure cooker to speed them up.