* XML (extensible markup language) is a method for putting structured data in a text file. Structured data includes spreadsheets, address books, financial transactions, technical drawings, etc. Programs that produce such data often also store it on disk, enabling use either as a binary format or a text format. XML offers a set of guidelines for designing text formats for such data in a way that produces files that are easy to generate and read by a computer.
* XML looks somewhat like HTML. Like HTML (hypertext markup language), XML makes use of tags (words bracketed by ?<' and ?>‘) and attributes (of the form name = ?value’). But while HTML specifies what each tag and attribute means, XML uses tags only to establish limits between pieces of data and leaves the interpretation of the data completely to the application that reads it.
* XML is text, but it isn’t meant to be read. XML files are text files because that format allows programmers to debug applications and fix files more easily.
* XML is a family of technologies. There is XML 1.0, the specification that defines what tags and attributes are, but around XML 1.0, there is a growing set of optional modules that provide sets of tags and attributes, or guidelines for specific tasks such as Xlink, Xpointer and Xfragments.
* XML creates large files. Because XML is a text format, XML files are nearly always larger than comparable binary formats.
* XML is new, but not that new. Development of XML started in 1996, and it has been a W3C standard since February 1998. Before XML, there was SGML, developed in the early 1980s, and HTML, developed in the early 1990s. Source: Bert Bos, W3C