XML stands for Extensible Markup Language and it is a language that defines a set of rules for encoding documents in a format that can be read by both people and computers. There is a set of specifications called XML 1.0, which is produced by W3C, and several other specifications for the language so far. XML is used fairly frequently to exchange and interpret data over the internet. The primary use for XML seems to be to send data back and forth from one browser to one web site and vice versa, while standard HTML is used for other more common tasks.
There have been criticisms of XML as being overly complicated and it’s rules and specifications overly verbose. As the specifications and rules are updated for the future there should be more uses for XML. Some note worthy projects based on XML and already in use are RSS, Atom, SOAP and XHTML.