In its broadest sense, 'cereal' is a very general word that covers all types of edible grasses - rice, maize, barley, oats, millet, buckwheat and so on. We can sometimes also use the word 'grain' to refer to these crops. 'Cereal' can also be used to refer to a cereal-based breakfast food. Even manufactured breakfast foods, such as cornflakes, Cheerios etc, can be called 'cereal'. 'Porridge' refers to a cooked dish made from flaked, rolled or ground oats boiled in water/milk, a typical breakfast dish originating in Scotland. 'Oatmeal' is the American term corresponding to more or less the same dish. If you say 'porridge', an English speaker would understand it to mean porridge made from oats. But if you want to describe the Russian dish made from rice, you can certainly call it 'rice porridge'. That would be a good term to use, and even people who aren't familiar with the dish would understand what it is. Likewise, if you wanted to describe a similar dish made from buckwheat, you could call it 'buckwheat porridge'.

Cereal is a a general name for food made with grains. For example, cold breakfast cereal. Porridge is a dish. It is made by heating up a cereal (rice, oat, buckwheat, etc) in water or milk. Oatmeal is oat porridge.

Porridge refers specifcally to a hot food made by boiling oatmeal with water and/or milk, traditionally a breakfast food. Oatmeal is ground oats.Oats is a cereal crop that grows well in poorer soils and tolerates bad weather, it has a low gluten content. Cereal refers generally to grass like crops, such as wheat, rye, barley and oats, it is also used to refer to breakfast foods specially prepared and made to be eaten with milk

Cereal, is a generic for a whole range of grasses (including rice and oats). It is also the term used in the UK for all the various packaged/sugary/child focused breakfasts that come in a box. porridge, is a generic term for a range of grass seeds boiled up and say for a thick vegtable soup. In the UK its core meaning is oat flake and water/milk boiled up (served with anything from salt through to honey). Famous as the stuff that makes for strong Scottish people. Oats is also an animal feed, though in its raw state oatmeal, is what results when you mill oats into a granular powder, so unlikely to be used to make a porridge, it is added to various breads andd baked products to bring the healthy benefits of oats to products that have had their healthy elements removed :-) you can say rice porridge, but you are more likely to say "rice pudding" in the UK, which is where rice and milk have been baked together with a little sugar. Famous as a cheep way of keeping children happy it takes white rice (all the goodness removed). Buckwheat is not a cereal, it is a fruit seed rather than a grass seed. I'm sure you can use "porridge" with it.