What is the difference between a URN and a URL?
Tim Rohaly From RFC 2396 "Uniform Resource Identifiers" (http://www.faqs.org/rfcs/rfc2396.html):
A URN differs from a URL in that it's primary purpose is persistent labeling of a resource with an identifier. That identifier is drawn from one of a set of defined namespaces, each of which has its own set name structure and assignment procedures.In other words, while a URL is a location of a resource, a URN is a globally-unique name for a resource. A resource can have more than one URL, for example if a document is available via HTTP or FTP on multiple web servers, but a resource can only have one URN. See the RFC for more detail.