XSLT is the W3C's expression language for describing locations within XML documents. Tools (such as Xalan) exist to allow executing XPaths directly as simple searches, but XPath is also a basic technology used by many of the other evolving XML standards and tools... XSLT and XPointer most obviously (since XPath is actually a shared product of the groups working on those two standards).
For official (formal) details, see http://www.w3.org/TR/xpath