Here is a reasonable explanation. For the most part, URL and URI are essentially the same thing. But whereas a URL is like an address, a URN is like a
name, and you need to use a
URN resolution service to look up the address (i.e., the URL/URI) that goes with it. Since such services aren't widely available, URNs aren't much more than a curiosity right now.
[ March 26, 2004: Message edited by: Ernest Friedman-Hill ]