Footnotes and endnotes are both ways of adding extra bits of information to your writing outside of the main text. Think of them like verbal asides, only in writing. You can use footnotes and endnotes to add side comments to your work or to cite other publications like books, articles, or websites. The only difference between footnotes and endnotes is where they appear in your document.
As the name suggests, footnotes are attached to the bottom of the page containing the sentence they correspond to. Endnotes, on the other hand, are added to the end of a section or document. Which one you should use in your writing depends on your personal preference or—if you’re writing for school or work—your organization’s publication standards.