From the software's FAQ list:
	So what is Mod Layout?

	Mod layout wraps a page, by saying this I mean that it can place
	information at both the beginning and the end of a document. This
	allows you do create a single look and feel throughout a website
	without using server side includes to "automagically" add content
	in either of these two locations. You can use it to add standard
	disclaimers to all of the pages on a server or to place banner
	adds on the top of all pages (I know, banner adds, ick...). I've
	even seen people use the header and footer to place the entire
	contents of a given server inside of tables. There are many
	possibilities.

WWW: http://www.tangent.org/mod_layout/