XML: Addressing_and_Querying
XPath is an XML Path language defined by the W3C for addressing parts of an XML document, designed to be used by both XSLT and XPointer.XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document. XML Pointer Language (or XPointer) defines a language for locating any element in a document. It is a special extension to a URL that allows it to access points deep within an XML document.XPointer connects to the end of a URL by the hash symbol (#), such as url#XPointer. XPointer is derived from XML Path Language (or XPath). XQL is a query language that uses XML as a data model, and it is very similar to XSL Patterns.XQL is not a version of the W3C's XML Query Language. However, both are XML-based query languages. From XQuery 1.0: An XML Query Language (W3C Working Draft 07 June 2001):"XML is an extremely versatile markup language, capable of labeling the information content of diverse data sources including structured and semi-structured documents, relational databases, and object repositories. A query language that uses the structure of XML intelligently can express queries across all these kinds of data, whether physically stored in XML or viewed as XML via middleware. Because query languages have traditionally been designed for specific kinds of data, most existing proposals for XML query languages are robust for particular types of data sources but weak for other types. This specification describes a new query language called XQuery, which is designed to be broadly applicable across all types of XML data sources." | 
