[URL,URN,URI,URC]

URI

URI = Uniform Resource Identifier

There are two types of URIs: URLs and URNs

See RFC 1630: Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web
and
RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax

URL

URL = Uniform Resource Locator

See RFC 1738: Uniform Resource Locators (URL)

<protocol>:// <host> [:<port>] [<path> [? <query>]]

URN

URN = Uniform Resource Name.

URNs define XML schemas.

urn:<namespace>:<string>

URC

URC = Uniform Resource Citation

URC provide a set of attribute/value pairs that describe properties of URIs like authorship, publisher, copyright etc.