Please refer to the following URL to know the differences between URL and URI,http://onlydifferencefaqs.blogspot.in/2012/08/difference-between-url-and-uri.html

A URL (Uniform Resource Locator) is the address of some resource on the Web. A resource is nothing but a page of a site.

URI is a generic term. URL is a type of URI.A URI is a unique identifier to usually a namespace.
Though it looks like a URL but it doesn’t have to necessarily locate any resource on the web.