From the W3C HTML 4.0 working draft:
    Every resource available on the Web -- HTML document, image, video clip, program, etc. -- has an address that may be encoded by a Universal Resource Identifier, or "URI".

    URIs typically consist of three pieces:
    1. The naming scheme of the mechanism used to access the resource.
    2. The name of the machine hosting the resource.
    3. The name of the resource itself, given as a path.