A Uniform resource identifier (URI) is a string of characters used to identify a name of a resource to interact with over a network, usually the World Wide Web, using specific protocols. Schemes that specifies a concrete syntax and associated protocols define each URI. The most commonly used URI is the uniform resource locator (URL), frequently referred to as a web address.
Protocols like “http”, “https”, “ftp”, “mailto”, “crid” or “file” are also URIs (and like HTTP address, are also the subsets of URI called a URL).
Another form of URI is the Uniform Resource Name (URN). A URN is a URI that identifies a resource by its name in a particular namespace. A URN is a form of URI that has an “institutional persistence,” that means its exact location may change from time to time, but some agency will still be able to find it.