Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). Originally designed to meet the challenges of large-scale electronic publishing, XML is also playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere.

Extensible Markup Language The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language for creating special-purpose markup languages, capable of describing many different kinds of data. In other words XML is a way of describing data and an XML file can contain the data too, as in a database. It is a simplified subset of Standard Generalized Markup Language (SGML). Its primary purpose is to facilitate the sharing of data across different systems, particularly systems connected via the Internet. Languages based on XML (for example, Geography Markup Language (GML), RDF/XML, RSS, Atom, MathML, XHTML, SVG,Klip and MusicXML) are defined in a formal way, allowing programs to modify and validate documents in these languages without prior knowledge of their form.

Features of XML

XML provides a text-based means to describe and apply a tree-based structure to information. At its base level, all information manifests as text, interspersed with markup that indicates the information's separation into a hierarchy of character data, container-like elements, and attributes of those elements.

XML makes no prohibitions on how it is used. Although XML is fundamentally text-based, software quickly emerged to abstract it into other, richer formats, largely through the use of datatype-oriented schemas and object-oriented programming paradigms (in which the document is manipulated as an object). Such software might treat XML as serialized text only when it needs to transmit data over a network, and some software doesn't even do that much. Such uses have led to "binary XML", the relaxed restrictions of XML 1.1, and other proposals that run counter to XML's original spirit and thus garner an amount of criticism.

Strengths

Some features of XML that make it well-suited for data transfer are: * its simultaneously human- and machine-readable format; * it has support for Unicode, allowing almost any information in any human language to be communicated; * the ability to represent the most general computer science data structures: records, lists and trees; * the self-documenting format that describes structure and field names as well as specific values; * the strict syntax and parsing requirements that allow the necessary parsing algorithms to remain simple, efficient, and consistent. XML is also heavily used as a format for document storage and processing, both online and offline, and offers several benefits: * its robust, logically-verifiable format is based on international standards; * the hierarchical structure is suitable for most (but not all) types of documents; * it manifests as plain text files, unencumbered by licenses or restrictions; * it is platform-independent, thus relatively immune to changes in technology; * it and its predecessor, SGML, have been in use since 1986, so there is extensive experience and software available.

The extensible Stylesheet Language (XSL) is a family of languages which allows one to describe how files encoded in the XML standard are to be formatted or transformed. There are three languages in the family:

* XSL Transformations (XSLT): an XML language for transforming XML documents

* XSL Formatting Objects (XSL-FO): an XML language for specifying the visual formatting of an XML document

* the XML Path Language (XPath): a non-XML language used by XSLT, and also available for use in non-XSLT contexts, for addressing the parts of an XML document.

These three specifications are available in the form of W3C Recommendations.

XSL Formatting Objects

XSL Formatting Objects are less widely supported. Most, if not all, current implementations are only partial. FOP, from the Apache project, can render a large portion of the XSL formatting objects specification to PDF and other output formats. The PassiveTeX package is another implementation that uses TeX to convert the output of an XSL-FO stylesheet to PDF.

Other file formats are supported to various degrees:

* PostScript
* SVG
* MIF
* PCL
* text files

Scalable Vector Graphics (SVG)

Scalable Vector Graphics (SVG) is an XML markup language for describing two-dimensional vector graphics, both static and animated, and either declarative or scripted. It is an open standard created by the World Wide Web Consortium.

Overview
SVG allows three types of graphic objects:

* Vector graphic shapes (e.g. paths consisting of straight lines and curves, and areas bounded by them)
* Raster graphics images / digital images
* Text

Graphical objects can be grouped, styled, transformed and composited into previously rendered objects. Text can be in any XML namespace suitable to the application, which enhances searchability and accessibility of the SVG graphics. The feature set includes nested transformations, clipping paths, alpha masks, filter effects, template objects and extensibility.

SVG drawings can be dynamic and interactive. The Document Object Model (DOM) for SVG, which includes the full XML DOM, allows straightforward and efficient vector graphics animation via ECMAScript or SMIL. A rich set of event handlers such as onmouseover and onclick can be assigned to any SVG graphical object. Because of its compatibility and leveraging of other Web standards, features like scripting can be done on SVG elements and other XML elements from different namespaces simultaneously within the same web page. An extreme example of this is a complete tetris game implemented as an SVG object, found here. (The link requires an SVG enabled browser.)

If storage space is an issue, SVG images can be saved with gzip compression, in which case they may be called "SVGZ files". Because XML contains verbose text, it tends to compress very well and these files can be much smaller. Often however the original vector-file (SVG) is already smaller than the rasterised version.