What is XML ?

Key Points of XML

• XML is not a programming language. XML is a Content Markup Structure that separates layout from data 
• XML works with almost every programming language 
• You create the tags and hierarchy within your XML yourself, the elements are not predefined 
• XML gives programmers a way to universally access data from one application to the next 
• XML is used for hundreds of custom markup languages including our beloved XHTML 
• XML is the most universally used method of separating content from layout

How XML Works

XML files have tags inside of them that we put there and give names to. XML tags can be nested into parent/child relationships if needed. Then as an application developer we use our favorite scripting language to create parsers to read and display the XML data in any way we see fit. We use the XML file's tag structure to know how to treat each piece of data when it is being parsed into our application by our favorite programming language.

Data is stored, read, and moved in XML, not directly displayed. The applications we create that work with XML files are the vehicles that parse and display the XML content. Applications we create can access, read, parse, change, and display the information in XML documents. An XML file by itself without an application to read and parse it, is a useless thing.

Application developers use many different programming languages and interfaces to access and display the data in XML documents (PHP, ASP, CFM, XSL, etc...). Which is where the term "XML Driven" comes from. If your application uses an XML file as a data structure, it is called an "XML Driven Application". I use PHP and Actionscript 3.0 frequently to access XML files that deliver content to my apps that are waiting to read and parse them. I also use PHP to create dynamically rendered XML data formats for my various apps to parse. I will show you examples of this later in the XML lessons and examples here.

A Little More Insight

XML is an acronym for "Extensible Markup Language". In web technology, "extensible" describes something that is designed so that users or developers can expand or add to its capabilities. Extensibility and ease of use are the primary reasons why XML is so popular today. Knowing what extensible means we can say "XML is a Markup Language that can easily be used and expanded by application developers". Without XML, XHTML as you and I know it today would not exist.

XML is derived from Standard Generalized Markup Language (SGML), and was introduced with the guidance and endorsement of the World Wide Web Consortium (W3C) in 1996.

XHTML(which most of us use when we create a web page) is an application of XML. XHTML brought more structure to HTML web documents, and can be parsed using standard XML parsers. Standard HTML has a loose parser with less structure. HTML was transformed into XHTML, so it can use an XML parser.

XML is one of the most widely used means of delivering content between various applications and technologies. XML can be used for everything from powering a Flash video playlist, to parsing an entire custom markup language.