URI (Uniform Resource Identifier)

In computing, a Uniform Resource Identifier (URI) is a compact string of characters used to identify or name a resource on the Internet. The main purpose of this identification is to enable interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols. URIs are defined in schemes defining a specific syntax and associated protocols.

Relationship to URL and URN

A URI may be classified as a locator (URL) or a name (URN), or both. A Uniform Resource Name (URN) is like a person's name, while a Uniform Resource Locator (URL) is like his or her street address. The URN defines an item's identity, while the URL provides a method for finding it.

A typical URN is the ISBN system for uniquely identifying books. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites unambiguously a specific edition of Shakespeare's play “Romeo and Juliet”. In order to gain access to this object and read the book, one would need its location: a URL address. A typical URL for this book is a file path, like file:///C:/MyDocuments/RomeoAndJuliet.pdf, identifying the electronic book saved in a local Windows PC hard disk. So the purposes of URNs and URLs are complementary.

Source: https://en.wikipedia.org