A Uniform Resource Identifier (URI), formerly Universal Resource Identifier, is a unique sequence of characters that identifies an abstract or physical resource, such as resources on a webpage, mail address, phone number, books, real-world objects such as people and places, concepts. URIs are used to identify anything described using the Resource Description Framework (RDF), for example, concepts that are part of an ontology defined using the Web Ontology Language (OWL), and people who are described using the Friend of a Friend vocabulary would each have an individual URI.
URIs which provide a means of locating and retrieving information resources on a network (either on the Internet or on another private network, such as a computer filesystem or an Intranet) are Uniform Resource Locators (URLs). Therefore, URLs are a subset of URIs, ie. every URL is a URI (and not necessarily the other way around). Other URIs provide only a unique name, without a means of locating or retrieving the resource or information about it; these are Uniform Resource Names (URNs). The web technologies that use URIs are not limited to web browsers. (Full article...)