XPath stands for XML Path Language. It uses a non-XML syntax to provide a flexible way of addressing (pointing to) different parts of an XML document. It can also be used to test addressed nodes within a document to determine whether they match a pattern or not.
XPath is mainly used in XSLT, but can also be used as a much more powerful way of navigating through the DOM of any XML-like language document, such as HTML and XUL, instead of relying on the document.getElementById method, the Node.childNodes properties, and other DOM Core features.
XPath uses a path notation (as in URLs) for navigating through the hierarchical structure of an XML document. It uses a non-XML syntax so that it can be used in URIs and XML attribute values.
Note: Support for XPath varies widely; it's supported reasonably well in Firefox (although there are no plans to improve support further), while other browsers implement it to a lesser extent if at all. If you need a polyfill, you may consider js-xpath or wicked-good-xpath.