The Uniform Resource Identifier (URI) is a series of characters formulated to uniquely identify a resource, most commonly on the Web and enable interaction with it via common protocols such as HTTP, FTP etc. The URI specification is maintained by W3C who also have documented the relationship between the different forms of addressing used on the Web. URNs and URLs are forms of URI. URIs are increasingly used to identify linked data resources where literal text values originating in a bibliographic record are replaced by URIs linking to database record identifiers (e.g. when replacing the text of an author’s name with a URI linking to the VIAF record for the author).
Uniform Resource Locators (URLs), both identify and locate a Web resource via inclusion of a protocol syntax, domain name and the name of the file within which a resource resides. A URL does not ensure a link to a resource is maintained if it is moved within its repository.
A Uniform Resource Name (URN) is a type of URI and is a standard, persistent and unique identifier for digital resources on the Internet. To link to the resource from the URN, a resolver service is required.