What is the difference between a URI, a URL and a URN?

Question

People talk about URLs, URIs, and URNs as if they\'re different things, but they look the same to the naked eye.

W

Answer 1

A small addition to the answers already posted, here's a Venn's diagram to sum up the theory (from Prateek Joshi's beautiful explanation):

And an example (also from Prateek's website):