XML is self-defining data. It is used to store, exchange, automatically process, error check, and display data to user-defined agreed standards. Because it is platform neutral it is fast becoming the IT industry standard. Companies in different industries using completely different computer hardware and software use xml to exchange information. Web browsers use it to completely separate data from its presentation yet still allow a well-crafted, complex web page. Its association of data with meaningful structure means that it is a very powerful language.

A stylesheet is used with a web browser to provide a visual representation of an xml file. By associating a stylesheet with an xml file the xml author completely splits the data content (the xml) from the presentation aspects (the stylesheet). This separation of the two aspects means that the data can be completely changed and the web page will still have the same appearance. Conversely the data may stay the same but be shown in a completely different way simply by altering the stylesheet.

An XML Document Type Definition (DTD) declares what can, what can not, and what must be included in its associated xml file. The xml documents DTD is checked by a web browser or other validating mechanism to ensure that the structure of the xml file is absolutely correct.