This digest provides an overview of XML (Extensible Markup Language), a markup language used to construct World Wide Web pages. Topics addressed include: (1) definition of a markup language, including comparison of XML with SGML (Standard Generalized Markup Language) and HTML (HyperText Markup Language); (2) how XML works, including sample tags, metadata, and use of XML by the GEM (Gateway to Educational Materials) Project; (3) customizing XML for individual needs, including the DTD (Document Type Definition), i.e., a formal set of markup elements developed as a standard for professionals in a particular field; and (4) making XML work on the Web, including limitations of older browsers. (Contains 12 references.) (MES)