A URI is simply a Web identifier, like the strings starting with http or ftp that you often see on the World Wide Web. Anyone can create a URI, and the ownership of URIs is clearly delegated, so they form an ideal base technology on top of which to build a global Web. In fact, the World Wide Web is such a thing: anything that has a URI is considered to be “on the Web.” Every data object and every data schema/model in the Semantic Web must have a unique URI.
A Uniform Resource Locator (URL) is a URI that, in addition to identifying a resource, provides a means of acting upon or obtaining a representation of that resource by describing its primary access mechanism or network location. For example, the URL http://www.doodhpatti.com is an URI that identifies a resource (DoodhPatti.com’s home page) and implies that a representation of that resource (such as the home page’s current HTML code, as encoded characters) is obtainable through HTTP from a network host named www.doodhpatti.com.
A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. You can use a URN to talk about a resource without implying its location or how to dereference it.