XML (eXtensible Markup Language) is a specification developed by the World Wide Web Consortium (W3C).it is very flexible text format derived from SGML (ISO 8879). Originally designed to meet the challenges of large-scale electronic publishing, XML is also playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere. It is a markup language much like HTML used to describe data. In XML, tags are not predefined. A user defines his own tags and XML document structure like Document Type Definition (DTD) , XML Schema to describe the data. Hence it is self-descriptive too .There is Nothing Special About XML It is just plain text with the addition of some XML tags enclosed in angle brackets. In a simple text editor, the XML document is easily visible .
Document Type Definition :Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes. A DTD can be declared inline inside an XML document, or as an external reference.
XML Schema :XML Schema is an XML-based alternative to DTDs. It describes the structure of an XML document. The XML Schema language is also referred to as XML Schema Definition (XSD).
"Well Formed" :XML document is a document that conforms to the XML syntax rules.
"Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD)
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