XHTML: XHTML, or Extensible HyperText Markup Language, is a family of computing languages meant to mark up HTML, in order to make it fit XML formatting standards.
Whereas HTML, particularly HTML 4, is viewed as an open, freeform language, XML is often seen as a much more rigid rule-based language. By marking up HTML to fit XML formatting as XHTML, developers can more effectively create code that sticks more closely to certain guidelines, increasing data interoperability between different types of hardware and software. This is particularly useful when looking to design a webpage or web application that can effectively be used on different types of mobile devices.
One of the biggest benefits of XHTML can be found in its modular design. XHTML can effectively be broken up into different segments -- or modules -- meaning that a piece of XHTML script used in one application can easily be located, copied, and reused in another. This useful feature of the language can both decrease the time from concept to implementation during a development project and help to keep coding clean.
XHTML was updated to version 1.1 three-years ago, but a new version, XHTML5, is currently being developed in order to provide an XML mark up option for the increasingly popular HTML5.