XML
Short for Extensible Markup Language, XML is a simple, very flexible system for defining data formats. It was derived from SGML (ISO 8879). Originally designed to meet the challenges of large-scale electronic publishing, XML is also playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere. XML based RSS and Atom feeds have become highly popular. If the programmer has the XML definition for a collection of data (often called a "schema") then they can create a program to reliably process any data formatted according to those rules.