What is the difference between a URI and a URL?

URLs are a subset of all URIs. From RFC 2396 "Uniform Resource Identifiers" (http://www.faqs.org/rfcs/rfc2396.html):

The term "Uniform Resource Locator" (URL) refers to the subset of URI that identify resources via a representation of their primary access mechanism (e.g., their network "location"), rather than identifying the resource by name or by some other attribute(s) of that resource.