Extensible Markup Language [XML] is a markup language which lets someone create their own custom markup for a page; it can be used to make some web pages. About fifteen years ago XML was all the rage--many developers thought it was going to replace HTML. However, XML was tricky, difficult to learn and never really caught on for website design. The rise of WordPress cancelled the need for would-be bloggers to learn XML in order to set up a blog. Search Marketers need to know that XML is the language with which sitemap.xml files are created.