Oatmeal is a product made by processing oats. In North America, oatmeal means any crushed oats, rolled oats, or cut oats used in recipes such as oatmeal cookies. It is also a name for a breakfast cereal made by cooking the oats. All oatmeal is porridge, but not all porridge is oatmeal.
Porridge made from oatmeal is also called oatmeal or oatmeal cereal. Porridge can be made with oats, rice, barley, cornmeal, brown rice, or basically any grain that is cooked, usually in water or milk. Similar dishes made with other grains or legumes often have other names, such as groat, polenta, grits, owsianka, or kasha. Since porridge was used as prison food for inmates in the British prison system, 'doing porridge' became a slang term for a sentence in prison.
Gruel is similar to porridge, but is made without milk and has a very thin consistency.
A spurtle is a Scottish kitchen tool used for stirring porridge.
Bonus - Pease porridge made from dried peas is another traditional English and Scottish porridge. It inspired the Mother Goose rhyme "pease porridge hot, pease porridge cold, pease porridge in the pot, nine days old.