In general, we know that a URI is “something” like a URL but we don’t properly understand the relationship in between. Moreover, we usually don’t know what a URN is.
If you are in this case, you will probably be interested in the following.
URLs (Uniform Resource Locators) are the most well-known identifiers and specify the location of a resource.
From a URL, I access the matching resource because I know exactly where to find it, I have its address.
Examples of URL:
URNs (Uniform Resource Names) are also identifiers, but they do not identify the location of a resource. They only help for identification (unique name). In other words, we know “what?” but not “where?”.
There are many examples of URN in everyday life. For example, ISBNs are URN. From an ISBN, I am able to determine what the book is, but I could not tell where to find it. The social security number is another example (unique within a country); it identifies a person’s record without knowing where it is located.
Examples of URN:
URL and URN are specializations of a URI (Uniform Resource Identifier) to identify a resource either by location, or a name, or both (usually the URL contains the name of the resource and therefore the URN).
Here is a Gmail URL pointing to a specific message:
141707ce154b142f is the identifier of this email (URN). This identifier is included in the URL in order to access it.