Parse the difference between URI and URL _ basics

Today, while looking at struts configuration, found that a problem is to look at the configuration file, sometimes the URL is sometimes a URI, so that I am puzzled, to the Internet to check a circle, the explanation is ambiguous, simply their own summary, only on behalf of individual understanding, because of my knowledge is limited, there may be mistakes, But I think that's the way it should be.

Summarized as follows:

1. Shorthand:
URI (Uniform Resource Identifier) uniform Resource identifier;
URL (Uniform Resource location) Uniform Resource Locator (or Uniform Resource Locator);
URN (Uniform Resource Name) uniform Resource naming.

2.URI Comparison with URLs

A.
A URI is a relatively broad concept, a URL is a URI, is a subset of the URI naming mechanism, you can say that the URI is abstract, and specifically to use the URL to locate resources.

B.
Each resource on the web, such as pictures, documents, videos, and so on, is positioned by the URI, where the so-called positioning refers to the specific path that the resource on the Web holds on the server relative to the host server.

C.
A URL is a string used on the Internet to describe an information resource file, used on client programs and servers to locate the information the client needs to connect to the server, not only to locate the information resource, but also to define how to find the resource.

3. My vernacular understanding
URI is a resource positioning mechanism, it is relatively general positioning of resources, not limited to the client and server, and the URL is positioned on the internet all resources, as long as the resources on the Internet, have a unique URL.