An Introduction to XML
(Excerpt from "XML in a Nutshell" by Elliotte Rusty Harold & W. Scott Means)
XML, the eXtensible Markup Language, is a standard for document markup introduced by the W3C. It defines a generic syntax for marking up data with simple, human-readable tags. It provides a standard format for computer documents that is flexible enough to accommodate domains as diverse as websites, electronic data interchange, vector graphics, object serialisation, family trees, real estate listings, remote procedure calls, voice mail systems and much more.
You can write your own programs to interact with and modify the data in XML documents. If you do, you can draw on a wide range of free libraries in various programming languages that can read and write XML, leaving you free to concentrate on your program’s requirements. You can also use existing software, such as web browsers and text editors, to work with XML documents. Some tools work with any XML document. Others are designed to support a particular XML application in a specific domain, such as vector graphics, and are not especially useful in other areas. In every case, however, the same underlying syntax is used, even when it is deliberately hidden by more user-friendly tools or restricted to a single application.