URI — Uniform Resource Identifier
URL — Uniform Resource Locator
URN — Uniform Resource Name
URC — Uniform Resource Citation
The traditional view:
A URI is either a URL or a URN (it could have been a URC, which refers to the metadata associated with a resource instead of the resource itself; but the URC never really took off).
A URL identifies a resource in terms of its location. Thus, http://www.google.com identifies the resource as reachable using http + IP address of http://www.google.com.
A URN identifies a resource, but does not convey location or other metadata associated with the resource. Thus, isbn:1-23-456789-0 identifies a book, but does not give any metadata information about the resource.
The current view:
The set of URIs is partitioned into subsets called subspaces.
a) The http example above represents the http subspace within the URI space; http is called a URI scheme.
b) The isbn example above is now represented as urn:isbn:1-23-456789-0, and urn is a subspace within the URI space. isbn is a namespace within the URN subspace.
c) URL refers to the subspaces which identify a resource via its location.
References:
— http://www.w3.org/TR/uri-clarification/
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