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The purpose of a DTD (Document Type Definition) is to define the legal building blocks of an XML document. A DTD defines the document structure with a list of legal elements and attributes.
XML Schema is an XML-based alternative to DTD. An XML schema describes the structure of an XML document. The XML Schema language is also referred to as XML Schema Definition (XSD). XML Schemas are extensible to future additions, are richer and more powerful than DTDs, are written in XML, support data types and support namespaces.
The XML DOM defines a standard way for accessing and manipulating XML documents. The XML DOM views an XML document as a tree-structure.All elements can be accessed through the DOM tree. Their content (text and attributes) can be modified or deleted, and new elements can be created. The elements, their text, and their attributes are all known as nodes.
XPath is a major element in W3C's XSLT standard - and XQuery and XPointer are both built on XPath expressions. XPath uses path expressions to select nodes or node-sets in an XML document.
XQuery is a language for finding and extracting elements and attributes from XML documents, works like SQL, built on XPath expressions and uses FLWOR ( "For, Let, Where, Order by, Return") command structure.
XSLT stands for XSL Transformations. XSL stands for EXtensible Stylesheet Language, and is a style sheet language for XML documents.