The terms uniform resource identifier (URI) and uniform resource locator (URL) are often used interchangeably. Unfortunately, these two terms describe slightly different concepts.

A URI is used to identify something on the World Wide Web. There are two types of URIs:
  • Uniform Resource Name (URN): URNs basically state what something is, but do not have information on how to access it.
  • Uniform Resource Locator (URL): URLs contain the location of something and tell the client program (usually a browser) how to access it.
Simply put, a URN is the name of something and the URL is the name and address.

For example, the URL tells your browser that a file called example1.html can be accessed through the example domain using the HyperText Transfer Protocol (HTTP). This means that your browser can request that file and display it for you using HTTP.

A URL is a type of URI, along with URNs. Most people will never have to worry about calling a URL a URI by mistake. In fact, in most cases they will be correct in the same way as someone calling a poodle a dog is correct. For people involved in the actual coding of applications, however, the difference is important because not every URI is a URL (just as not every dog is a poodle).