XML is "extensible" because, unlike HTML, the markup symbols are self-defining. XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for how to create a document structure.
XML is designed especially for Web documents and provides a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. It allows designers to create their own customized tags, enabling the definition, transmission, validation, and interpretation of data between applications and between organizations.
XML documents fall into two broad categories:
i. Data-centric – XML is used as a data transport
Example: Sales orders, patient records, directory entries, and metadata records
ii. Document-centric – XML is used for its SGML-like capabilities, reflecting the structure of particular classes of documents
Example: Books with chapters, user manuals, newsfeeds and articles incorporating explicit metadata in addition to the text