Footers are found at the bottom of the page (in the margin area) and often consist of centred page numbers.
A header is the detail at the top of the page, usually in smaller type than the main body of text. You’ll often find the book title on the left-hand page and the chapter title on the right-hand. Sometimes, however, the chapter heading is on the left and the current section heading on the right.
Headers are standard in nonfiction but appear less frequently in fiction (fewer than 50% of books in a spot check I did on my bookshelves). So, if you’ve written a novel, you don’t need to add them. Headers are completely doable but you may lose your sanity during the process so please, run for the hills and just make do with a nice little page number in the footer area. If you’ve written nonfiction, pretend you didn’t just read that and remember, I’ll be with you all the way!