XML
XML is short for Extensible Markup Language. It is designed not only to store data, but to automatically provide metadata through semantic syntax including tags and attributes. Furthermore this is accomplished while maintaining a human-readable format, unlike more optimized database storage which is binary and must be manually queried using software to browse the information. Here is an example of some XML.
XML is the basis of [[XHTML]], MathML, SVG, and many other markup languages that have been extended for a specific use, often specified using a DTD. Another common application is the use of XML feeds for content syndication between a website and a user, or between two web servers.
XML has a very specific set of rules that determine if it is well-formed XML. In order to maintain its usefulness, many programs, such as web browsers, refuse to process malformed XML. This is why using valid XHTML is so important, versus earlier implementations of [[HTML]] in which validity was easily forgiven.