In formal writing the footnote refers to any material that is not your own. If you borrow, paraphrase, or quote from someone else you can cite the material in a footnote. Each citation style (MLA,APA, Chicago) has its own rules on whether to use footnotes or endnotes, but as a rule of thumb, footnotes are used for ease of reading. If you write a long essay and have to insert some additional information that doesn't fit into the flow of your essay but should be mentioned, place it in a footnote for the reader to see. If you are writing for college, ask your professor whether he/she would like footnotes or endnotes. For instance, if your citation style is MLA ( Modern Language Association), your paper should technically be formatted containing endnotes in addition the Works Cited list, but many professors ask specifically for footnotes. I think this trend has perhaps to do with the fact that many professors now request electronically submitted papers and endnotes do not require scrolling to the end of the document.