Uniform Resource Identifier (URI)
URI is a compact string of characters used to identify or name a resource. The main purpose of this identification is to enable interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols. URIs are defined in schemes defining a specific syntax and associated protocols.
Here's a URI example: http://en.wikipedia.org/wiki/Uniform_Resource_Identifier A URI may be classified as a locator (URL) or a name (URN) or both.
A Uniform Resource Name (URN) is like a person's name, while a Uniform Resource Locator (URL) is like their street address. The URN defines something's identity, while the URL provides a method for finding something. Essentially, "what" vs. "where". For more information, see http://www.w3.org/TR/cooluris/