This article shows in a concise and graphical way the difference between URIs and URLs.
A URI is an identifier of a specific resource. Like a page, or book, or a document.
A URL is special type of identifier that also tells you how to access it, such as HTTPs, FTP, etc.—like https://www.google.com.
So, an example of a URI which is clearly not a URL is an ISBN code for a book.