Short for Extensible Markup Language, XML is a specification developed by the W3C. XML is intended to be a standard for creating markup languages which describe the structure of data. It is not a fixed set of elements like HTML, but rather, it is a metalanguage, or a language for describing languages. XML enables authors (i.e. programmers) to define their own tags.
XML has become widely used because it provides a very rich system to define complex documents and data structures. Typical uses include representations of invoices, molecular data, news feeds, glossaries, inventory lists, real estate properties, etc. As long as a programmer has the XML definition for a collection of data (often called a "schema") then they can create a program to correctly and consistently process any data formatted according to those rules.
- Document Type Definition (DTD)
- Extensible Stylesheet Language (XSL)