These are the newly harvested oats. They still have their hulls and stalks attached.
These are just separated from the hulls and stems and are called groats. This is the whole grain kernel of the oat. You can most likely find these in a health food store and they take a long time to cook.
Steel Cut Oats
These are simply groats cut into two or three pieces by a sharp metal steel blade. These cook a little faster than whole groats. They are also known as Irish oatmeal.
The Scottish folk stone-grind these oats making them into various sizes of broken bits. This is said to be a creamier oatmeal than steel cut oats.
Rolled Oats (regular)
Rolled oats are sometimes called old-fashioned oats. They are made by steaming and then rolling the groats into flakes. This stabilizes their healthy oils so they can stay fresh longer. It also creates a larger surface area so the oats cook faster than the above styles.
Rolled Oats (quick or instant)
These are simply the above rolled oats only they are rolled thinner and/or steamed longer. The nutrition does not vary but the texture does. Some people may say these are too mushy.
A whole-grain flour used for cooking, baking, thickening.
This is not a whole grain as it doesn’t contain the germ or endosperm of the oat.