XPointer, the XML Pointer Language, defines an addressing scheme for individual parts of an XML document.
XLinks point to a URI (in practice, a URL) that specifies a particular resource.
The URI may include an XPointer part that more specifically identifies the desired part or element of the targeted resource or document.
XPointers use the same XPath syntax you're familiar with from XSL transformations to identify the parts of the document they point to, along with a few additional pieces.