Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. XML (Extensible Markup Language) is a set of rules for encoding documents electronically. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards.XML''s design goals emphasize simplicity, generality, and usability over the Internet.It is a textual data format, with strong support via Unicode for the languages of the world. Although XML''s design focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services. There are many programming interfaces that software developers may use to access XML data, and several schema systems designed to aid in the definition of XML-based languages.