Difference Between URL and URI
stackoverflow.com
stackoverflow.com
An 'nntp' URL specifies a specific message in a specific group on a specific server, hence it always has an authority part. It's also indicating to use the NNTP protocol to retrieve the message.
A 'news' URI, on the other hand, identifies a specific message by its message ID, and leaves it to the user agent to choose a protocol and server.
(The distinction isn't quite that clear cut, as 'news' URIs can specify a server, making them more like URLs, but I think it conveys the difference.)
http://www.rfc-editor.org/rfc/rfc3986.txt
From section 1.1.3:
An individual scheme does not have to be classified as being just one
of "name" or "locator". Instances of URIs from any given scheme may
have the characteristics of names or locators or both, often
depending on the persistence and care in the assignment of
identifiers by the naming authority, rather than on any quality of
the scheme. Future specifications and related documentation should
use the general term "URI" rather than the more restrictive terms
"URL" and "URN" [RFC3305].The classic hierarchy ("URLs and URNs are distinct subsets of URIs") wasn't as neat and useful as originally thought, so "it's just URIs now, with their own scheme-specific qualities" is the modern thinking, as reflected by this passage from RFC3986.
The modern answer to "What's the difference between a URI and URL?" is thus very nearly 'Mu.'
(I find sycophantic behaviour a terrible trait)