What are footnotes?

Some articles on footnotes:

Windows Help - Source Files and Compilation
... Each topic has a series of footnotes that contain information for the help compiler # footnotes contain the topic ID (used to create links to that topic) ... $ footnotes contain the topic name as it displays in the table of contents, index, and other locations ... K footnotes contain keywords for the index ...
Ciaphas Cain - Character
... Vail must also add occasional footnotes to some of his recollections to point out that Cain has just glossed over something another man might take pride in ... Vail's footnotes in the second novel indicated Cain was probably one of the finest swordsmen in his region of the galaxy ... Vail's footnotes indicate that, despite her research into the subject, she could find no official documentation on where he was born or what his childhood was like ...
Princeton Footnotes
... The Princeton Footnotes are a traditional all-male a cappella group from Princeton University in Princeton, NJ ... The Footnotes are a student-run, semi-professional performance group ... The Footnotes came in third in the National Collegiate A Cappella Championship Semi-Final in 2000 ...