Uniform Resource Identifier
URI, short for Uniform Resource Identifier, is an Internet protocol element. It was codified by the IETF as RFC 2396, based on earlier proposals from Tim-Berners Lee.
A URI is a short string of characterss that conform to a certain syntax. The string indicates a name or address that can be used to refer to an abstract or physical resource.
The URI syntax is essentially a URI scheme name like "http", "ftp", "mailto", "urn", etc., followed by a colon character, and then a scheme-specific part. The semantics of the scheme-specific part are determined by the specifications that govern the schemes, although the URI syntax does force all schemes to reserve certain characters for special purposes, without always saying what those purposes are.
A URI Reference is a URI combined with an optional fragment identifier, appended to the end of the URI and preceded by "#". Strictly speaking, a URI Reference that includes a fragment identifier is not a URI, although this distinction is often not maintained in informal usage (it should be maintained in protocol documents). The fragment identifier identifies a fragment of the resource that is identified by the preceding URI.
A URL, Uniform Resource Locator, is a URI that is acting as a locator, i.e. to retrieve a resource (as opposed to just referring to it). Note that this depends on the context of use. For example, when a http: URI refers to an XML namespace, it is not being used as a URL, even though it may be possible to use the same string of characters as a URL by typing it into a web browser (hopefully, resulting in documentation for the namespace).
URN, Uniform Resource Name, is a URI that use the "urn" scheme, and does not connote availability of the identified resource.
To "resolve" a URI means either to convert a relative URI reference to absolute form, or to dereference a URI or URI reference by attempting to obtain a representation of the resource that it identifies. The "resolver" component in document processing software generally provides both services.
If a URI reference, when resolved to absolute form, is determined to be equivalent to the URI of the document containing the reference, then the document processing software is supposed to use its current representation of the document to satisfy the resolution; a new representation is not to be fetched.
see also internet, History of the Internet, websiteExample Absolute URIs
Example URI references
Typically, there is a URI that identifies the document that contains the URI reference. The URI reference, if relative, is considered to be relative to the document's URI, except in cases when some other part of the document, or some out-of-band means, has somehow been used to establish a different "base URI" to resolve against.
