What Is XML?
XML and HTML are complimentary markup languages used for the presentation of data to the WWW. HTML is a language that describes how information is displayed on the screen in your web browser and XML describes how data is organized, stored and retrieved from memory.
XML is simply text. There are no calculations done nor any type of display of information taking place. XML files simply assign tags to data that will be meaningful to the application that needs access to a specific set of data.
By writing XML files for specific tasks and for specific types of data, many data files can be imported to an application for manipulation. The resulting data can then be displayed based on a HTML style sheet.
Data information that is collected, stored and used by dynamic web pages is defined by XML. XML is used to let the web page find, manipulate and display the information of interest.
XML is a language that conforms to specific rules for syntax. An integral part of the XML documents is the XML processor. The processor reads the XML document and is able to define the format of the complex document.
HTML Versus XML
HTML is the notation for Hyper Text Markup Language. The HTML language governs the attributes of a text file that fills web pages. It controls a page's text color, background color, font, size of text, images and any other aspect of the page that may be important. HTML also defines how content that is not text is displayed on the page, such as music, sound bytes and video.
XML, on the other hand, is designed to transport data from one place to another, in most cases in reference to files and applications. The display of data is not part of the tasks for which XML is used. No tags are defined in XML; therefore, you must supply your own. XML was conceived to be self-descriptive. You must specify within your files all the relationships between tags, therefore you set the semantics. Each file written in XML is thus independent of any other file.
Requirements of an XML document
Many are familiar with the markup delimiters used in HTML. With HTML, there are defined tags and semantics. These define the beginning and ending of a particular attribute that is to be applied to the document. Not only do they define the textual attributes but tags can also pull content from other locations on the Internet where they are stored.
XML is a meta-language that describes the tags and semantics for a markup language. Each XML formatted file will define the semantics and relationship of tags for the application for which it is being used. In the basic definition, XML is a restricted form of the SGML, Standard Generalized Markup Language. The XML file describes tags and the relationship of tags between each other and with the data associated with the particular application.
Defining the HTML and XML Languages
XML is defined by the W3C, World Wide Web Consortium. This is an international group that is developing standards for web content and protocols for the prolonged growth of the WWW.
The Consortium is the driving force behind the development of languages and utilities that aid the WWW community as a whole in making information available and useful to users of the Internet. An example of their work can be found in the previous and current versions of HTML.