You are right, it is complicated. This is because people want to do so many different things with them. It becomes even more complex when you add in StyleRef Fields which let them change for every page while Word sees it as the same header/footer.

Each document has at least one section. In the last paragraph mark of that section (or the section break) is the coding for three headers and three footers (even if you never use or see them). You can have multiple sections on a page. This is something you never need to know unless you start creating sections, but every time you change the margins (not indents, margins), layout, or column structure in part of a document, you are adding one or more sections.

There are a lot of different things that can be done using this tool. Here is the recap to help keep things sorted out: Header/Footer Settings Recap

(and, you hoped you had asked a simple question )