Footnote
A note printed at the bottom of the page, keyed to a superscript number in the text, that carries either a citation or a comment too tangential for the body.
In note-based styles — Chicago notes-bibliography, Turabian, OSCOLA, Bluebook, AGLC — the footnote is the citation. The sentence carries only a superscript number, and the full source details live at the foot of the page. The first note for a source gives everything; later notes are shortened.
Footnotes also carry substantive content: a caveat, a translation, a pointer to a debate you are not having. Disciplines differ sharply on how welcome that is. History and law expect it; the sciences almost never use footnotes for anything.
Endnotes are the same mechanism collected at the end of the chapter or book instead of the bottom of the page. Which one you use is usually a publisher or department requirement rather than a style rule.