Uniform Addressing

Uniform Resource Identifier (URI) generic terms for all textual names and addresses. A URI can be a URL or URN or URC.

A uniform resource identifier is a string of characters that unambiguously identifies a particular resource. To guarantee uniformity, all URIs follow a predefined set of syntax rules, but also maintain extensibility through a separately defined hierarchical naming scheme (e.g. http://).

Such identification enables interaction with representations of the resource over a Network, typically the world wide web using specific protocols. Schemes specifying a concrete syntax and associated protocols define each URI. 

The most common form of URI is the uniform resource locator (URL) frequently referred to informally as a web address. More rarely seen in usage is the uniform resource name (URN) which was designed to complement URLs by providing a mechanism for the identification of resources in particular namespaces.

Uniform Resource Name (URN) a URI that has an institutional commitment to availability. A particular scheme intended to identify resources.  

A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN may be used to talk about a resource without implying its location or how to access it. For example, in the International Standard Book Number (ISBN) system, ISBN 0-486-27557-4 identifies a specific edition of Shakespeare's play Romeo and Juliet. The URN for that edition would be urn:isbn:0-486-27557-4. However, it gives no information as to where to find a copy of that book.

Uniform Resource Locator (URL) the set of URI schemes that have explicit instructions on how to access the resource over the internet. 

A Uniform Resource Locator (URL) is a URI that specifies the means of acting upon or obtaining the representation of a resource, i.e. specifying both its primary access mechanism and network location. For example, the URL http://example.org/wiki/Main_Page refers to a resource identified as /wiki/Main_Page, whose representation, in the form of HTML and related code, is obtainable via the Hypertext Transfer Protocol (http:) from a network host whose domain name is example.org.

A URN may be compared to a person's name, while a URL may be compared to their street address. In other words, a URN identifies an item and a URL provides a method for finding it.

Uniform Resource Characteristic (URC) provides meta information