URI (thing)

Uniform Resource Indicator.

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

            URI
             ^
             |
         +---+---+
         |       |
        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).