XML -- (eXtensible Markup Language) A widely used system for defining data formats. XML provides a very rich system to define complex documents and data structures such as invoices, molecular data, news feeds, glossaries, inventory descriptions, real estate properties, etc. As long as a programmer has the XML definition for a collection of data (often called a "schema") then they can create a program to reliably process any data formatted according to those rules.

XPFE -- (Cross Platform Front End) A suite of technologies used to create applications that will work and look the same on different computer operating systems. A widely used XPFE application is the Netscape web browser in version 7 and later. The primary technologies used in creating XPFE applications are Javascript, Cascading Style Sheets, and XUL.

XUL -- (eXtensible User-interface Language) A markup language similar to HTML and based on XML. XUL used to define what the user interface will look like for a particular piece of software. XUL is used to define what buttons, scrollbars, text boxes, and other user-interface items will appear, but it is not used to define how those item will look (e.g. what color they are). The most widely used example of XUL use is probably in theMozilla web browser, where the entire user interface is defined using the XUL language.