"In e107 a theme contains a set of modular code
containing XHTML (eXtensible HyperText Markup Language) and CSS
(Cascading Style Sheets). In its most basic explanation, XHTML
allows us to take a text document, create a link to other
documents, and the eXtensible part of the language allows you to
create your own tags describing data. Thus a program like e107 can
use these tags to extract the information it needs. "