Java

Before going into URL and URI,
you need to know some background. Do you ever thought about, who decides what is URL? and what is URI? or who is the authority for URL, URI and such naming conventions?

W3C and IETF

There are two separate bodies W3C and IETF. The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. The specifications for URI and URL are defined by W3C.
It was founded and headed by Sir Tim Berners-Lee. He is one of the greatest scientist living now. He created this www model of server and client architecture, a web server serving web pages through network and client browsers reading it. He did it first when he was with CERN. He created the world’s first web page http://info.cern.ch/ . He was also in HTML 2.0 working group of IETF. So it is very appropriate for W3C to define URI and URL.
Internet Engineering Task Force (IETF) is an open international community working on Internet related standards. In general it addresses issues of Internet protocols.
In particular W3C defines the web, html specifications and related information. IETF defines IP, TCP, or DNS, for security at any of these levels; with SMTP or NNTP protocols.
So the stake for definition for URI and URL is with W3C. But as there is only a thin line between these organisation’s work they tend to cross each other. In some place IETF gives dissimilar definition for URL and URI.
If you read through the huge volume of journals available in web for this topic, you can sense that experts :-( are using URI and URL synonymously. Which is causing all these confusion among the web community about URI, URL and URN.

URI

An URI identifies a resource. It is a locator. It includes a URI scheme, authority, path, query and fragment by syntax. For example, http: is a URI scheme.
Syntax of URI based on RFC 3986

foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
| _____________________|__
/ \ / \
urn:example:animal:ferret:nose

URL

The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network “location”).

URN

The term “Uniform Resource Name” (URN) is used to identify a resource independent of its location. Example urn:ISBN:1-23-432536-5

Summary of differences between URI and URL

• A URI was either a URL or a URN.
• URL is a subset of URI. It identifies a resource using one of the URI schemes.
• URN is a subset of URI. It identifies a resource independent of its location.

Whenever you have a doubt that, whether something is a URL or URI then use URI as a term to identify it. Since URI is a super set of URL.