There are three kinds of oats used for oatmeal.
Before the you get the oats in the store they all go through some processing. All oats have the outer hull removed and are toasted leaving groats. You are technically eating groats not oats everytime you have oatmeal. This still has the bran attached and is quite healthy for you.
Steel Cut Oats: Are groats simple cut up into smaller pieces, approximately the size of a sesame seed. This type of oat meal takes the longest to cook. About 45 min, but also produces the heartiest type of oatmeal. This is by far my favorite way to have oaks.
Stone Ground Oats: These are processed just like steel cut oats, they are just ground into smaller pieces. This cuts down cooking time significantly, about 20 min.
Rolled oats: The groats are steamed and then rolled out. A bed dryer then dries the oats to 11% moisture. This decreases the cooking time even more, cutting it down to about 10 minutes.
Instant Oats: They are the same as rolled oats, just rolled even thinner. Most have oats that have been ground into a powder increase the thickness of the porridge If left on their own they make a very thin oatmeal, however it cooks quite quickly.
Nutritionally speaking there isn't much difference between all the types. Steel cut and stone ground are slightly less processed, and they make a heartier porridge so I really prefer them, however I don't always have time to make them so I will often go with instant.
Are Steel Cut Oats Healthier?
How oats are Processed