Oats, or avina sativa, are a highly nutritious grain which provide good amounts of slow released carbohydrate for sustained energy. They are also high in a type of soluble fibre which is particularly good at helping to lower our cholesterol levels called beta-glucan. There are different forms in which we can buy oats and add them to our nutrition plan, so let's briefly summarise the different types.
Cultivated oats are first dehusked to remove a very tough and silicified husk, and then usually rolled to split the grain, which gives us what we know as raw rolled oats, the more familiar presentation which we use in porridge or oatmeal, in muesli or as an ingredient in many foods like flapjacks or cereal bars. You'll notice instant oats in the shops which are thin pre-cooked rolled oats for instant cereals which may have been milled into smaller flakes.
Oat groats are minimally processed and only the outer hull is removed. They're very nutritious, but are chewy and need to be soaked and cooked for a long time. It is from groats that rolled and instant oats are formed.
Steel-cut oats, which are also known as Irish or Scotch oats, are groats which have been cut into smaller pieces and are chewier than rolled oats, and are often preferred for hot oatmeal cereals and muesli. Oats can also be milled into oat flour which in cookery is not as versatile as wheat flour, but is an alternative for some recipes.
All types of oats are great to include as all are high in beta-glucan and are low on the glyceamic index, meaning that they provide a more slowly, sustained release of energy and help keep you full up. So it doesn't really matter what types of oats you include in your diet, just make sure you include some foods which are based on oats.