URL og URI

http://www.webmasterworld.com/forum21/3878.htm%23msg8

uri =      A Universal Resource Identifier is a formatted string
            that serves as an identifier for a resource, typically
            on the Internet. URIs are used in HTML to identify the
            destination of hyperlinks. URIs in common practice
            include Uniform Resource Locators (URLs)[URL] and
            Relative URLs [RELURL].

A URI can be further classified as a locator, a name, or both.  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.

Altså er URL en delmængde af en URI (URI er mere generel end URL).
Formel beskrivelse af URI http://www.ietf.org/rfc/rfc2396.txt
Formel beskrivelse af URL ftp://ftp.rfc-editor.org/in-notes/rfc1738.txt