What is the difference between a URL and a URL? 

Let’s start by expanding the the abbreviations and throwing in URN

URI – Universal Resource Identifier

URL – Universal Resource Location

URN – Universal Resource Name

In syllogymistic terms URI is the category and URL and URN are subcategories.  So all URLs are URIs but not all URIs are URLs and all URNs are URIs but not all URIs are URNs.

If the above makes your head swim in a semantic soup as it does mine then maybe an example will help: Wikipedia’s article on URI’s can be found at http://en.wikipedia.org/wiki/URI. The name of the article is “Uniform resource identifier” it uses  Romeo and Juliet (isbn:0-486-27557-4) which can be found at http://www.gutenberg.org/catalog/world/readfile?fk_files=1444611 as an example.

Resource URI URL URN
http://en.wikipedia.org/wiki/URI X X X
en.wikipedia.org/wiki/URI X   X
Romeo and Juliet      
urn:isbn:0-486-27557-4 X    
http://www.gutenberg.org/catalog/world../readfile?fk_files=1444611 X X  

http://en.wikipedia.org/wiki/URI – is all three because it identifies a unique resource and gives the location and the name (“URI”).

en.wikipedia.org/wiki/URI – Does not include the how to access the resource so lacks full location information.

Romeo and Juliet – Even though we know the story it could refer to a book or a play or a movie or many things so it’s not unique.

urn:isbn:0-486-27557-4 – Even though it is unique it doesn’t give the name or the location.

http://www.gutenberg.org/catalog/world/readfile?fk_files=1444611 – Has an unique id and gives the location but not the name.  If you want to argue 1444611 is name then it’s also a URN.

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