URL stands for Uniform Resource Locator, URI for Uniform Resource Indicator and URN for Uniform Resource Name.
URL indicates a very specific information about where the resource can be located and URN refers to the name of the resource. It indicates that ALL URLs are subsets of URI but not vice-versa.
A URL can be referred to as a URI but the term URL has been deprecated by IETF and W3C though it is in wide use in world wide web.
RFC3986 states URI as a locator, a name or both while URL refers to the identification of a resource by describing its network location. Check RFC3986 http://www.ietf.org/rfc/rfc3986.txt for URI: Generic Syntax guidelines and on the sample schemes used in URIs.
RFC2648 sheds some light on the terms URI, URN (http://www.ietf.org/rfc/rfc2648.txt)
RFC2141 http://www.ietf.org/rfc/rfc2141.txt states that ” Uniform Resource Names (URNs) are intended to serve as persistent, location-independent, resource identifiers.” and add that ALL URNs have a following syntax
<URN> ::= “urn:” <NID> “:” <NSS>
TIP: It is always a good practice to refer RFC’s when it relates to World Wide Web related clarifications to get an indepth knowledge on the subject.
