Uniform Resource Identifier / Uniform Resource Locator. A URL is the http://… web address of some page or resource. When a URL is used in linked data as the identifier for some object, it is not strictly a locator for the object (e.g. http://dbpedia.org/page/Paris is the location of a document about Paris, but not of Paris itself), so in this context it is referred to as a URI.
URI - Uniform Resource Identifier. A string that uniquely identifies virtually anything, including a physical building or more abstract concepts such as colours. It may or may not be resolvable on the web.
URL - Uniform resource locator. A global identifier commonly called a web address. A URL is resolvable on the web. All HTTP URLs are URIs; however, not all URIs are URLs.