URLs, URNs and URIs

The most frequently encountered of these three acronyms is (probably) URL, then URI, and finally (and much less frequently, if at all) URN, though this may be changing.

1. Uniform Resource Locator (URL) A Uniform Resource Locator (URL) is, as its name suggests, is a uniform (or "standard") way of referring to the location of a web document (or, more generally, to the location of a web resource of whatever kind). Naturally, therefore, the fully qualified domain name of the host machine on which the resource is located forms an integral part of the URL for that resource. However, it is not enough to know where the resource is located. Once must also know, and be able to specify, how the resource will be accessed (i.e., the method, or "scheme", that will be used to access the resource). After all, not every web "resource" is just a page to be displayed. So, a URL has the following form:

   scheme:address_of_resource
   

   The address_of_resource, on the other hand, itself has the following form:

   //fully_qualified_domain_name/path_from_document_root/name_of_resource
   

   Thus, if you enter something very typical, like

   http://cs.smu.ca/jobs/2006/current.html
   

   into your browser's "address window" and click on Go or press Enter, you are saying you want to retrieve the document current.html using the http protocol scheme, from the server whose fully qualified domain name is cs.smu.ca. The forward slash (/) immediately following cs.smu.ca refers to the directory on that host which is the host's document root. The rest of the path (starting from the document root, wherever it might be located, and something we cannot determine from the URL) is jobs/2006/, and the final item is the name of the actual desired resource, current.html.

   Sometimes you will see a URL that looks like this:

   http://cs.smu.ca/~porter/jobs/2006/current.html
   

   In a case like this the tilde symbol (~) indicates that the name porter is the name of a user on the host system and that cs.smu.ca/~porter is the "home directory" (or "personal document root") for this particular user on this server.
2. Uniform Resource Name (URN) A URN is a name that has the same form as a URL, but may not identify an actual location on the Internet. So, a URN can be used to talk about something without implying its location or indicating how to get at the particular resource referenced by the name.
3. Uniform Resource Identifier (URI) A URI is a more general concept than either a URL or a URI. The "contemporary" viewpoint is that URLs and URNs are both "context-dependent aspects of URI and rarely need to be distinguished" (to quote Wikipedia). In fact, it is suggested in the same Wikipedia article that the term URL may be falling into disuse, since it is "rarely necessary to distinguish between URIs and URLs", and the more "user-friendly" term "web address" is now more frequent in any case.