David Olsen retweeted this great article by Daniel Miessler on the difference between URLs, URIs, and URNs. You should read it in full, but he summarises the issue as:
The RFCs are ancient, poorly written, and not worth debating until they’re updated. A URI is an identifier. A URL is an identifier that tells you how to get to it. Use the term that is best understood by the recipient.
I’ve resisted the push to refer to URLs as URIs, despite being called out for it a few times by those in the ackchyually brigade. It largely comes down to that last point in the above list; something that nerds like me all too often ignore.
As another example, it used to bug me when people referred to HTTPS certificates as SSL, when technically we’ve been on TLS for two decades. Or referring to random number generators as pseudo-random.