In the United States, we rarely use the word "porridge."
The word "cereal," without qualification, almost always means dry, packaged "breakfast cereal," eaten cold with milk on it. Examples include corn flakes, shredded wheat, and many kinds of brand names (Cheerios, Fruity Pebbles, Cocoa Puffs, etc.). It also includes the more "natural" cold cereals like granola and muesli. "Hot cereal" is the usual term for those that are eaten hot--by far the most popular is oatmeal; others include Cream of Wheat (farina), and "grits."
As a technical term in agriculture or ethnogeography, "cereal" can mean "various species of starchy grains grown for food." It's named for Ceres, the Roman goddess of agriculture. Getting truly off-topic, the totally different words "cereal" and "serial" are pronounced exactly the same way and are a frequent subject of puns.
When I was a child, I encountered the word "porridge" in two places: one of the "Mother Goose rhymes," and the story of "Goldilocks and the Three Bears." I had no idea what it was and had to have it explained to me.
"Pease porridge hot,
Pease porridge cold,
Pease porridge in the pot
Nine days old.
Some like it hot,
And some like it cold,
And some like it in the pot
Nine days old."
I assume that "pease porridge" was made of peas. I think the rhyme is still a part of popular culture in the U.S. Certainly it's the source of the movie title, "Some Like It Hot."