Okay, so I write this from memory. Check Wikipedia for a lot more accurate and elaborate versions of web history. This is just to give you a quick humorous hint and how things go sometimes.
When TBL aka Tim-Berners Lee hacked up his WWW thing in 1991, he absorbed a tendency that had been fluttering around on Usenet to write FTP addresses in a uniform style. Back then people used to write /email@example.com. TBL came up with the absolutely brilliant idea to have such hostname/path combinations in a standardized way and to prefix them with the protocol name. Thus was born the URL.
Some years later folks at either IETF or W3C complained that the URL was too much pointing to a certain host where a file resides, rather then describing the identity of the file. So, mentally, the Uniform Resource Identifier (URI) was born. However in several years of URI working groups nobody ever found a viable scheme to actually do this. The world was waiting for it and in the meantime just used a virtual URL which would redirect by HTTP code 302 to the nearest possible copy of the file. The closest we got was probably the eDonkey URI.
Anyway, the working group had to have some results, so whatever they figured out was subsequently declared however useful, so the URI was suggested as being a better term than the original URL. Even though nothing really had changed. So nowadays, URI is just the more formal way to say URL. Like saying centripetal rather than centrifugal or saying XMPP rather than Jabber.
There is a detail about PSYC's use of the "@" character. Places in PSYC have psyc://psyced.org/@welcome as their UNI. The "@" is however declared as 'reserved' in the URI RFC. Unless we are seriously mistaken, this means that a psyc: URI scheme definition would need to intentionally use that reserved "@" character and define it appropriate use. Not a problem, just a little uneven since the "~" character for people doesn't need that treatment.
- <lynX> We could of course use '*' here as well as we already do for XMPP and IRC URIs. There is some unbeatable logic to that. It just hurts a bit, as "@" is so nice. Is legacy conformance important enough to surrender PSYC's uniform beauty?
See Wikipedia:URL for more.