Tobias Schlitt and Jacob Westhoff have written up and article (and posted it over on Tobias' blog) that wants to help you understand XPath better - whether you're a novice or have been using it for a while.
This paper will give an overview on XPath an addressing language for XML documents. XPath is a W3C recommendation currently in version 1.0. XPath was created in relation to the XSL recommendation and is intended to be used with XSLT and XPointer. Beside that, XPath can be used in a variety of programming languages, commonly in combination with a DOM API.