A footnote is for a reference that documents or elucidates the primary text.5
Footnotes are distracting; use them sparingly at most, and it is best to avoid them completely. Standard bibliographical references are usually better placed in a bibliography at the end of a document instead of in footnotes throughout.
A footnote should complement or expand upon the primary text, but a reader should not need to read a footnote to understand the primary text. For a thorough discussion of footnotes, see The Chicago Manual of Style, which is published by the University of Chicago Press.