Corny issue.

Deci: "hoho, i saw your ever so detailed summary of the differences between a muesli and a granola bar! haha. Now how about cereals and cornflakes."

In response to Ernest's question, here's the answer!

Cereal: a grain used for food, such as wheat, oats, or corn

Cornflakes: a breakfast cereal consisting of toasted flakes made from corn.

So, conclusion: 

All cornflakes are cereals, but not all cereals are cornflakes. "Cornflakes" are a 'corny' subset of "cereals".

Hope that clears things up!