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.

One of the cantons of Switzerland. It is somewhat like a state in America, only the cantons of Switzerland have more freedom.

Uniform Resource Indicator.

It's a superset of URLs that also includes URNs.

         |       |
        URN     URL

So, in common terms: URIs are sort of like URLs, but they also include URNs. Which means, they include common web addresses and other common things you can probably access with a web browser, and names that refer to something that's somewhere but we don't really specify where, let the machine do the job (understandably the latter aren't used that much yet).

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