XML: Extensible Markup Language
XML (Extensible Markup Language) is a markup language similar to HTML, but without predefined tags to use. Instead, you define your own tags designed specifically for your needs. This is a powerful way to store data in a format that can be stored, searched, and shared. Most importantly, since the fundamental format of XML is standardised, if you share or transmit XML across systems or platforms, either locally or over the internet, the recipient can still parse the data due to the standardised XML syntax.
There are many languages based on XML, including XHTML, MathML, SVG, XUL, XBL, RSS, and RDF. You can also define your own.