stands for User Interface Markup Language
and is an application of XML
to describe user interfaces. Normally people think of using XML for describing documents or data. But as it is just a formalism it may be used for any kind of structured data. There are tools which convert an UIML representation to the representation for various GUIs (e.g. Java awt).
XUL (Xml User-interface Language) provides similar functionality as UIML and is used for the Mozilla web browser.