X.25 - In 1976 X.25 was recommended as the desired protocol by the International Consultative Committee for Telegraphy and Telephony (CCITT) called the International Telecommunication Union (ITU) since 1993. X.25 is a packet switched data network protocol which defines an international recommendation for the exchange of data as well as control information between a user device (host), called Data Terminal Equipment (DTE) and a network node, called Data Circuit Terminating Equipment (DCE). In other words, this protocol was used for wide area networks using the phone or ISDN system as the networking hardware. The X25 standard consists of 3 layers as follows: The packet layer, the link layer, and the network (or packet) layer. These three levels conform to the lower three layers of the OSI seven-layer reference model for open system interconnection. X.25 networks are still in use throughout the world, although mainly in developing countries. Layer 2 technologies such as frame relay, ISDN, ATM, ADSL, POS, and the layer 3 Internet Protocol have largely replaced the need for X.25 elsewhere. X.25 Overview
XHTML (eXtensible HyperText Markup Language) - The evolution of HTML has essentially stopped. Instead, HTML is being replaced by a new language, called XHTML. XHTML is in many ways similar to HTML, but is designed to work with the new eXtensible Markup Language, or XML, that will soon serve as the core language for designing all sorts of new Web applications, in which XHTML will be only one of many "languages". XHTML is designed to work with these other languages, so that different documents, in different languages, can be easily mixed together.
XML (eXtensible Markup Language) - XML is a standard for creating markup languages which describe the structure of data. It is not a fixed set of elements like HTML, but rather, it is like a language for describing languages. XML enables authors to define their own tags. XML is a formal specification of the World Wide Web Consortium. Official Site XML Tutorial
XSL (eXtensible Stylesheet Language) - XSL and is a language for expressing style sheets. An XSL style sheet is, like with CSS, a file that describes how to display an XML document of a given type. Separating the document's content and the document's styling information allows displaying the same document on different media (like screen, paper, cell phone), and it also enables users to view the document according to their preferences and abilities, just by modifying the style sheet.