Document addressing - uniform resource term information
This World Wide Web Consortium page provides information on the uniform resource family of terms that deal with addressing documents on the Web. There is information on Uniform Resource Identifier (URI), Uniform Resource Locators (URL), Uniform Resource Names (URN), Uniform Resource Citations (URC), and how they all work together.

Hypertext version of RFC 1630
Defines the syntax used by the World Wide Web initiative to encode the names and addresses of objects on the Internet.

Yahoo!'s URI page
Yahoo!'s directory of URI.

IETF's URI Working Group Page
Describes the IETF URI working group and includes contact and mailing list information, RFC revisions-in-progress, Internet drafts, new URL scheme proposals, RFCs, and links to related work.

RFC 2396
Request for Comments document for the Uniform Resource Identifiers (URI): Generic Syntax.