URI.

Overview of URI

URI are characterized by the following definitions:

Uniform

Uniformity provides several benefits:
　　it allows different types of resource identifiers to be used in the same context, even when the mechanisms used to access those resources may differ;
　　it allows uniform semantic interpretation of common syntactic conventions across different types of resource identifiers.
　　it allows introduction of new types of resource identifiers without interfering with the way that existing identifiers are used.
　　it allows the identifiers to be reused in many different contexts.

Resource
　　A resource can be anything that has identity.including an electronic document, an image, a service (e.g., "today's weather report for Los Angeles"), and a collection of other resources.

URI, URL, and URN

　　A URI can be further classified as a locator, a name, or both.  The

   term "Uniform Resource Locator" (URL) refers to the subset of URI

   that identify resources via a representation of their primary access

   mechanism (e.g., their network "location"), rather than identifying

   the resource by name or by some other attribute(s) of that resource.

   The term "Uniform Resource Name" (URN) refers to the subset of URI

   that are required to remain globally unique and persistent even when

   the resource ceases to exist or becomes unavailable.


ref:http://www.ietf.org/rfc/rfc2396.txt