XML is 'extensibile' - programmers create their own set of 'tags' depending on the specific needs of a document. XML is new, but the underlying technology has been under development for over a decade. XML is as powerful as SGML, but XML is simpler to use. The markup language provides the rules for putting the tags into the document. Here is an example of tags.
Tags enclose the text that they describe.
XML is a flexible markup language, yet for business applications like proposal authoring it is important to enforce rules and recognize bad data.