The development of the internet has not been a singular effort. Because of that, there is a lot of jargon that gets thrown around. Some terminologies that the public actually get acquainted with do not usually get used the way it was originally meant. As normal people do not really take the time and effort to look deeper into what the acronym meant and how it should be used, they just observe how the word is used when they heard it and use it as they see fit. This is comparable to how Xerox became a word synonymous to photocopying, simply because of the brands popularity. The problem with this is that it would often lead to confusion especially when technical people talk to non-technical ones.
One of the terminologies that caught on pretty quick is the acronym URL. URL stands for Uniform Resource Locator and it is supposed to be used only for identifiers that point out a location. In the general public, any hyperlink is called a URL even if it is not necessarily a URL.
The URI or Uniform Resource Identifier was created as the name for anything that points to a resource. This was then further subdivided into two general categories, the URL and the URN (Uniform Resource Name), with each one handling a group of naming conventions. The URN was meant to describe a group of URIs that contains only the name of the resource and not necessarily where it is located and how to get it. The URL as described above provides a location and the protocol that would be used to access the stored data.
To sum it up, the URL and URN are both a part of the bigger and more general URI. So it would be safe to call a hyperlink as a URI no matter what it points to, what the name is, and what protocol is used to access the data. The URL is simply a subgroup that is contained by the URI. It was only meant to point to the location of the resource with an indication of the protocol needed. There was no need to distinguish the two since most of the time using the term URL was correct. And due to the ever growing popularity of the term URL in the general public, the two has become almost interchangeable despite the inaccuracies. It is also worth noting though that the technical writing have begun the shift to using the term URI in favor of the term URL.
Learn more technical jargons.