A URI may be classified as a locator (URL) or a name (URN), or both.
A Uniform Resource Name (URN) is like a person's name, while a Uniform Resource Locator (URL) is like his or her street address. The URN defines an item's identity, while the URL provides a method for finding it.
A typical URN is the ISBN system for uniquely identifying books. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites unambiguously a specific edition of Shakespeare's play "Romeo and Juliet". In order to gain access to this object and read the book, one would need its location: a URL address. A typical URL for this book is a file path, like file:///C:/MyDocuments/RomeoAndJuliet.pdf, identifying the electronic book saved in a local Windows PC hard disk. So the purposes of URNs and URLs are complementary.
A URL is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL http://www.wikipedia.org/ is a URI that identifies a resource (Wikipedia's home page) and implies that a representation of that resource (such as the home page's current HTML code, as encoded characters) is obtainable via HTTP from a network host named www.wikipedia.org. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to access it. For example, the URN urn:isbn:0-395-36341-1 is a URI that, like an International Standard Book Number (ISBN), allows one to talk about a book, but doesn't suggest where and how to obtain an actual copy of it.
In technical publications, especially standards produced by the IETF and the W3C, the term URL has long been deprecated, as it is rarely necessary to distinguish between URLs and URIs. However, in nontechnical contexts and in software for the World Wide Web, the term URL remains ubiquitous. Additionally, the term web address, which has no formal definition, is often used in nontechnical publications as a synonym for URL or URI, although it generally refers only to "http" and "https" URL schemes.
Much of this discussion is taken from RFC3305, titled "Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations". This RFC outlines the work of a joint W3C/IETF working group that was setup specifically to normalize the divergent views held within the IETF and W3C over what the relationship was between the various "UR*" terms and standards. While not published as a full standard by either organization, it has become the basis for the above common understanding and has informed many standards since then.
The URI syntax is essentially a URI scheme name like "HTTP", "FTP", "mailto", "URN", "tel", "rtsp", "file", etc., followed by a colon character, and then a scheme-specific part. The syntax and semantics of the scheme-specific part are determined by the specifications that govern the schemes, although the URI syntax does force all schemes to adhere to a certain generic syntax that, among other things, reserves certain characters for special purposes, without always saying what those purposes are. The URI syntax also enforces restrictions on the scheme-specific part, in order to, for example, provide for a degree of consistency when the part has a hierarchical structure. Percent-encoding is an often-misunderstood aspect of URI syntax.
URIs and URLs have a shared history. The idea of a URL — a short string representing a resource that is the target of a hyperlink — was implicitly introduced in early 1990 in Tim Berners-Lee’s proposals for HyperText At the time, it was called a hypertext name or document name
Over the next three-and-a-half years, as the World Wide Web's core technologies of HTML (the HyperText Markup Language), , and Web browsers were developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and the more contentious Uniform Resource Name came to represent the latter.
During the debate over how to best define URLs and URNs, it became evident that the two concepts embodied by the terms were merely aspects of the fundamental, overarching notion of resource identification. So, in June 1994, the IETF published Berners-Lee's RFC 1630: the first RFC that (in its non-normative text) acknowledged the existence of URLs and URNs, and, more importantly, defined a formal syntax for Universal Resource Identifiers — URL-like strings whose precise syntaxes and semantics were dependent upon their schemes. In addition, this RFC attempted to summarize the syntaxes of URL schemes that were in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifiers.
With the publication of RFC 2396 in August 1998, the URI syntax became a separate specification , and most parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded. In the new RFC, the "U" in "URI" was changed to represent "Uniform" rather than "Universal", and only those portions of RFC 1738 that summarized existing URL schemes were not modified by RFC 2396.
In December 1999, RFC 2732 provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development of a number of draft revisions under the title rfc2396bis. This community effort, coordinated by RFC 2396 co-author Roy Fielding, culminated in the publication of RFC 3986 in January 2005. This RFC is the current version of the URI syntax recommended for use on the Internet, and it renders RFC 2396 obsolete. It does not, however, render the details of existing URL schemes obsolete; those are still governed by RFC 1738, except where otherwise superseded — RFC 2616 for example, refines the "http" scheme. The content of RFC 3986 was simultaneously published by the IETF as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol.
In August 2002, RFC 3305 pointed out that the term URL has, despite its ubiquity in the vernacular of the Internet-aware public at large, faded into near-obsolescence. It now serves only as a reminder that some URIs act as addresses because they have schemes that imply some kind of network accessibility, regardless of whether they are actually being used for that purpose. As URI-based standards such as Resource Description Framework make evident, resource identification need not be coupled with the retrieval of resource representations over the Internet, nor does it need to be associated with network-bound resources at all.
On November 1, 2006, the W3C Technical Architecture Group published On Linking Alternative Representations To Enable Discovery And Publishing, a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content.
A URI reference may take the form of a full URI, or just the scheme-specific portion of one, or even some trailing component thereof—even the empty string. An optional fragment identifier, preceded by "#", may be present at the end of a URI reference. The part of the reference before the "#" indirectly identifies a resource, and the fragment identifier identifies some portion of that resource.
In order to derive a URI from a URI reference, the URI reference is converted to "absolute" form by merging it with an absolute "base" URI, according to a fixed algorithm. The URI reference is considered to be relative to the base URI, although if the reference itself is absolute, then the base is irrelevant. The base URI is typically the URI that identifies the document containing the URI reference, although this can be overridden by declarations made within the document or as part of an external data transmission protocol. If a fragment identifier is present in the base URI, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process.
In web document markup languages, URI references are frequently used in places where there is a need to point to other resources, such as external documents or specific portions of the same logical document.
srcattribute of the
imgelement is a URI reference, as is the value of the
hrefattribute of the
SYSTEMkeyword in a DTD is a fragmentless URI reference.
hrefattribute of the
xsl:importelement/instruction is a URI reference, as is the first argument to the
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.
A URI reference may be considered to be a same-document reference: a reference to the document containing the URI reference itself. Document processing software is encouraged to use its current representation of the document to satisfy the resolution of a same-document reference; a new representation should not be fetched. This is only a recommendation, and document processing software is free to use other mechanisms to determine whether obtaining a new representation is warranted.
According to the current URI specification, RFC 3986, a URI reference is a same-document reference if, when resolved to absolute form, it is identical to the base URI that is in effect for the reference. Typically, the base URI is the URI of the document containing the reference. XSLT 1.0, for example, has a
document() function that, in effect, implements this functionality. RFC 3986 also formally defines URI equivalence, which can be used in order to determine that a URI reference, while not identical to the base URI, still represents the same resource and thus can be considered to be a same-document reference.
Same-document references were determined differently according to RFC 2396, which was made obsolete by RFC 3986 but is still used as the basis of many specifications and implementations. According to this specification, a URI reference is a same-document reference if it is an empty string or consists of only the "#" character followed by an optional fragment.
XML has a concept of a namespace, an abstract domain to which a collection of element and attribute names can be assigned. An XML namespace is identified by a character string, the namespace name, which must adhere to the generic URI syntax. However, the namespace name is not considered to be a URI because the "URI-ness" of strings is, according to the URI specification, based on how they are intended to be used, not just their lexical components. A namespace name also does not necessarily imply any of the semantics of URI schemes; a namespace name beginning with "http:", for example, likely has nothing to do with the  protocol. There has been much debate about this among XML professionals on the xml-dev electronic mailing list; some feel that a namespace name could be a URI, since the collection of names comprising a particular namespace could be considered to be a resource that is being identified, and since the Namespaces in XML specification says that the namespace name is a URI reference. The consensus seems to be, though, that a namespace name is just a string that happens to look like a URI, nothing more.
Initially, the namespace name was allowed to match the syntax of any non-empty URI reference, but the use of relative URI references was later deprecated by an erratum to the Namespaces In XML Recommendation. A separate specification was issued for namespaces for XML 1.1, and allows IRI references, not just URI references, to be used as the basis for namespace names.
In order to mitigate the confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL was developed, though the specification of RDDL (http://www.rddl.org/) has no official standing and has not been considered nor approved by any organization (e.g., W3C). An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. XML document authors were encouraged to put RDDL documents in locations such that if a namespace name in their document was somehow dereferenced, then an RDDL document would be obtained, thus satisfying the desire among many developers for a namespace name to point to a network-accessible resource.