The following information is an edited version of the URL guide that is provided by the NCSA Mosaic package. We believe that the service abstraction provided by the URL concept is a good one, and hope to provide more search and retrieval products under it that take advantage of this paridigm in the future.
What's a URL? A URL is a Uniform Resource Locator. Think of it as a networked extension of the standard filename concept: not only can you point to a file in a directory, but that file and that directory can exist on any machine on the network, can be served via any of several different methods, and might not even be something as simple as a file: URLs can also point to queries, documents stored deep within databases, the results of a finger or archie command, or whatever.
Since the URL concept really pretty simple ("if it's out there, we can point at it"), this beginner's guide is just a quick walk through some of the more common URL types and should allow you to be creating and understanding URLs in a variety of contexts very quickly.