What is the difference between URL and URI?

A URL (Uniform Resource Locator) is the address of some resource on the Web. A resource is nothing but a page of a site. There are other type of resources than Web pages, but that's the easiest conceptually.  

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.

URI is a generic term. URL is a type of URI.