XPath

 XPath provides a mechanism to address parts of an XML  document. XPath also provides basic facilities for manipulation of strings, numbers and booleans. XPath uses a compact, non-XML syntax to facilitate use of XPath within URIs and XML attribute values. XPath operates on the abstract, logical structure of an XML document, rather than its surface syntax. XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document.  In addition to its use for addressing, XPath is also designed so that it has a natural subset that can be used for matching (testing whether or not a node matches a pattern).

XPath models an XML document as a tree of nodes. There are different types of nodes, including element nodes, attribute nodes and text nodes. XPath defines a way to compute a string value  for each type of node. Some types of nodes also have names. XPath fully supports XML Namespaces. So, the name of a node is modeled as a pair consisting of a local part and a possibly null namespace URI.

XPath is the result of an effort to provide a common syntax and semantics for functionality to be shared between at least two other W3C specifications, XSL Transformations and XPointer. XPath is used in conjunction with each of these specifications.