A Uniform Resource Identifier (URI) is a compact string of characters
for identifying an abstract or physical resource. See IETF RFC2396 at 
http://www.ietf.org/rfc/rfc2396.txt?number=2396

Uniform Resource Identifiers (formerly UniformResourceLocator''''''s, or URLs--this term is still commonly used) are the lifeblood of the WorldWideWeb.

''URL and URI are different concepts. A URL gives you information on how you can access a resource, whereas a URI merely identifies a resource and does not in general give you any information on how you might access it (or even imply that you can access it in any meaningful way). For example, ''
 urn:schemas-microsoft-com:office:excel
''is the URI for the XmlNameSpace for some exchange format to do with MicrosoftExcel and this URI is not an URL.''

Many have the form:

 [protocol]:[//][hostname][:port]/[path/information][?query=[...]][&...]

'''protocol''' is one of http https file mailto news telnet gopher ftp plus some more obsolete protocols.

'''hostname''' is an IP address in decimal or decimal dotted form, or the name of a host on that can be resolved to an IP address.

'''port''' is the internet port to connect to.  If absent, the default port for the given '''protocol''' is used (e.g., port 80 for http). The hostname and port combo must end with a /.

'''path/information''' is somewhat dependent on the protocol, and sometimes is not present.  It's typically hierarchical.  The last part of the path is often used to indicate the type of data and what the client might do to it.  For example ".txt" indicates normal preformatted ASCII text and ".html" HTML.

'''?query=''' is the non-hierarchical segment, typically used as command arguments.

e.g.
 http://c2.com/cgi/wiki

 file://ftp.sendmail.org/pub/sendmail/

 mailto:garrod@ucsd.edu

See also http://usemod.com/cgi-bin/mb.pl?UniformResourceLocator.

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Actually, it's not that simple, and people shouldn't be writing URI parsers
if under the impression that it is.  See 
	http://www.faqs.org/rfcs/rfc2396.html
for details  --DanBarlow

''Specifically, a URL has location information contained in it, but URLs are only one kind of URI. This is an instance of OnlyKnowingBestKnownCase. -- RobertField''
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Expect this page to have a high lapse rate as the details get straightened out. URIs often only partially understood.
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CategoryAcronym CategorySemanticWeb CategoryDiscovery