URNs
URLs are just one form of reference, and are part of a wider standard called URI (Universal Resource Identifier). Apart from the URL standard, the other standard that URIs embrace is called the URN (Uniform Resource Name) standard.
Background
The URN concept is described in a specification called RFC 2141. While URLs describe where a resource is (using domain names, and file paths or specific queries), URNs describe what the resource is instead.
A URN has the advantage of being independent of the location of the resource, so it remains applicable even when the resource is moved, but has the consequent disadvantage that a look-up mechanism is required to resolve the URN into a URL whenever the resource is requested. This mechanism must be ...
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