XML, the Extensible Markup Language, is a W3C-endorsed standard for document markup. It defines a generic syntax used to mark up data with simple, human-readable tags. It provides a standard format for computer documents that is flexible enough to be customized for domains as diverse as web sites, electronic data interchange, vector graphics, genealogy, real estate listings, object serialization, remote procedure calls, voice mail systems, and more.
You can write your own programs that interact with, massage, and manipulate the data in XML documents. If you do, you'll have access to a wide range of free libraries in a variety of languages that can read and write XML so that you can focus on the unique needs of your program. Or you can use off-the-shelf software, such as web browsers and text editors, to work with XML documents. Some tools are able to work with any XML document. Others are customized to support a particular XML application in a particular domain, such as vector graphics, and may not be of much use outside that domain. But the same underlying syntax is used in all cases, even if it's deliberately hidden by the more user -friendly tools or restricted to a single application.