XML, or Extensible Markup Language provides definitive rules for the encoding of documents in a format which is both machine-readable and human-readable. It was developed because there was a recognised need for a persistent, simple and usable alternative to the much more inflexible HTML, which was being used for tasks it was not intended for, and were realistically beyond its scope.
XML was designed to simplify, and as such provides a basic syntax which standardised the separation of appearance, content and meaning within electronic documents. XML provides the machine with understanding, or at least the appearance of understanding; it tells the computer that John is a human, a male, lives in London, so the machine can then search under these terms and find the corresponding information. XML tells the computer that London is a city, is in the UK, is in Europe etc., and provides as much or as little detail as the writer would like. This detail then becomes available to a user when searching. In this sense XML is self-defining, and fully customisable, the writer tells the computer what something is, and the computer then has the ability to read that description and provide it for the user.
XML has allowed flat, scanned digital documents to become more dynamic and alive, by allowing searching within the text, and by allowing deeper and deeper layers of description to be utilised. The level of detail is defined by the person writing the XML. XML language can be used to share information accross different, diverse platforms and applications without the need for conversion; it is universal. XML is a persistent, usable, free to use, simple, self-defining, universal mark-up language which provides computers with semantic, human information.