What is a URN?
URN stands for Uniform Resource Name. URNs are location-independent pointers to a resource, which may be a single file or even different representations of the same content (e.g. a specific image in GIF, JPEG and PNG form). In most ways URNs can be used like URLs except that, when fetched, a smart browser should fetch them from the closest source. URNs and URLs are both types of Uniform Resource Identifiers, or URIs.
For more information read the following specifications: