Most applications allow you to use special symbols in the footer that represent changing values. For example, you can enter a symbol for the page number, and the application will replace the symbol with the correct number on each page. If you enter the date symbol, the application will insert the current date, which will change if necessary each time you print the document.
You can usually specify at least two different footers, one for odd-numbered pages (odd footer) and one for even-numbered pages (even footer).
A footer is sometimes called a running foot.