uri encompasses/superset of both url and urn (there was also urc that went no where). urls have a "scheme" (http/ftp/gopher/etc) to access the resource, while

uri encompasses/superset of both url and urn (there was also urc that went no where). urls have a "scheme" (http/ftp/gopher/etc) to access the resource, while urn is a persistent non-location based type of name identifier e.g. urn:ietf:rfc:2648