GuideML (Guide Markup Language) is a document markup language used on h2g2, and other bbc.co.uk communities that use the same "DNA" software.
GuideML is an application of XML standards. It mostly consists of a safe subset of XHTML with some extra tags for specific features of the software. The aim is to provide something which resembles HTML but is simpler to learn, and allows basically textual content to be formatted in a skinnable way. Early versions of the h2g2 software offered full HTML markup as an option, but this was removed for security reasons; thus only parts of HTML which are considered to exist in GuideML can now be used, with things such as JavaScript and externally hosted images being removed by the parser.
Some of the more elabourate features of GuideML are referred to as "gadgets" because they perform functions beyond straightforward formatting. These include the ability to format a list of user IDs into a drop-down list with the option of opening in the same or a different window; and the famous <VIEWER/> tag, which displays as the username of the person viewing the page, if they're logged in.
A proposal for a standardized "GuideML 1.0" was under discussion, and would have involved a specific subset of the currently supported tags being declared "official" parts of the language. This would have been an important change of approach, in that the current parser lets through any unrecognized tags, as long as they are XML-compliant, meaning that any XHTML understood by the viewer's browser is implicitly part of GuideML.