What is XML?
XML stands for eXtensible Markup Language. It is an extensible document language for specifying document content (i.e. data). Unlike HTML, XML does not contain pre-defined elements for describing the presentation of the document. For example, HTML contains predefined <table> and <font> tags that indicate how the data is to be presented. XML has none of that.
XML, on the other hand, is extensible. It allows the document author to define his own tags and assign meaning to them. The part of the XML document that accomplishes this is the Document Type Declaration (DTD). The DTD consists of a formal definition of the document type. It assigns names for element types and describes the valid syntax of the document.