W3C: URIs, URLs, and URNs

Report from the joint W3C/IETF URI Planning Interest Group
W3C Note 21 September 2001
This document addresses how URI space is partitioned and the relationship between URIs, URLs, and URNs;  describes how URI schemes and URN namespace ids are registered; ists some additional unresolved issues not considered by this paper; and presents recommendations.

Note: this document is fairly old but contains some recommendations that have not been overturned, as far as I can see.