URI is meant to identify a resource. A URI can be a URL or a URN. To explain further I will use an example.
Suppose you want to buy the book "Network Analysis" by Van Valkenburg off of Amazon. You can identify that book by its
URN: ISBN-10: 0136110959
URNs are still in the experimental stage. There is no infrastructure yet to convert the names into addresses. Most of the URIs you come across on the internet are URLs.