If something is fundamental, it is basic or intrinsic. In the context of "Let's learn the fundamentals," I think of the fundamentals as being elementary concepts. When something is foundational, then it provides support. Think of how a house is built upon a strong foundation to provide stability for building the structure on top of it.

A foundation is something to build on. If someone has the foundational concepts or basic knowledge of a language, or of mathematics, then you can go on to more advanced concepts and learning. Fundamental is a word that can be used in other ways. Fundamental truths of philosophy or a religion are the basic non-debatable beliefs a sect might have. There may be other debatable concepts where you can be tolerant of other members' viewpoints, but the fundamental core beliefs are agreed on by all members of the organization.

Sorry kat, your answer is great too. Just Gena got it answered first. So...