From http://www.w3schools.com (Copyright Refsnes Data)

XPath Tutorial

XPath is a language for finding information in an XML document. XPath is used to navigate through elements and attributes in an XML document.

XPath is a major element in the W3C's XSLT standard - and XQuery and XPointer are both built on XPath expressions.

So an understanding of XPath is fundamental to a lot of advanced XML usage.

XPath Reference

At W3Schools you will find a complete reference of all the built-in functions in XPath 2.0, XQuery 1.0 and XSLT 2.0.

XPath Introduction
This chapter explains what XPath is.

XPath Nodes
This chapter defines the different types of nodes in XPath and the relationship of nodes.

XPath Syntax
This chapter explains the XPath syntax.

XPath Axes
This chapter explains the XPath axes.

XPath Operators
This chapter lists the operators that can be used in XPath expressions.

XPath Examples
This chapter uses the "books.xml" document to demonstrate some XPath examples.

XPath Summary
This chapter contains a summary on what you have learned in this tutorial and a recommendation on what subject you should study next.