XPath is a language for defining parts of an XML document. It allows you to locate and operate on elements and attributes using path expressions that work like URLs or file system directories. XPath contains functions for selecting nodes, evaluating expressions, and extracting information from XML documents. It is used by several XML standards and technologies like XSLT, XQuery, XML Schema, and Schematron. The document provides an introduction to XPath basics like selecting elements, using attributes and namespaces, and built-in functions for strings, numbers, and booleans. Additional resources are listed for learning more about XPath.