The Uniform, Resource*, Identifier* (URI) is the cornerstone of the Web. More information is available at the W3C's Addressing web site.
Are you confused? (I don't blame you.) Dan Connolly presents URI terminology demystified -- quite a good piece, I think.
There are many different URI schemes and URI implementations.
Some of the top guns in the URI business have proposed IRIs. Leslie Daigle* has written something on the problems of internationalization.
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