A URI (Uniform Resource Identifier) is a string of characters that identifies a resource.
URI could be URL (Uniform Resource Locator), or URN (Universal Resource Name).
The first part of the URL specifies the protocol.
For example, here's a URL to a web page on the Internet: http://www.google.com/intl/en/about.html
A URN looks something like this: urn:foo:a123,456
xmlns="http://www.rntsoft.com"
elementFormDefault="qualified">