A hyperlink, or simply a link, is a reference in a hypertext document to another document or other resource. As such it would be similar to a citation in literature. However, combined with a data network and suitable access protocol, it can be used to fetch the resource referenced. This can then be saved, viewed, or displayed as part of the referencing document.
Hyperlinks are part of the foundation of the World Wide Web created by Tim Berners-Lee.
Hyperlinks in various technologies
Hyperlinks in HTML
Tim Berners-Lee saw the possibility of using hyperlinks to link every unit of information to any other unit of information over the Internet. Hyperlinks were therefore integral to the creation of the World Wide Web.
Links are specified in HTML using the <a> (anchor) elements.
Hyperlinks in XML
A special W3C Recommendation called the XML Linking Language, XLink, describes simple (i.e. as in HTML) and extended links for hyperlinking from, within, and between XML documents.
Hyperlinks in other technologies
Hyperlinks are used in PDF documents, word processing documents, spreadsheets, Apple's HyperCard and many others.
How hyperlinks work in HTML
A link has two ends, called anchors, and a direction. The link starts at the source anchor and points to the destination anchor. However, the term link is often used for the source anchor, while the destination anchor is called the link target.
The most common link target is a URL used in the World Wide Web. This can refer to a document, e.g. a webpage, or other resource, or to a position in a webpage. The latter is achieved by means of a HTML element with a "name" or "id" attribute at that position of the HTML document. The URL of the position is the URL of the webpage with "#attribute name" appended.
Link behaviour in web browsers
A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different colour, font or style. The behaviour and style of links can be specified using the Cascading Style Sheets (CSS) language.
In a graphical user interface, the usage of a mouse cursor may also change into a hand motif to indicate a link. In most graphical browsers, links are displayed in underlined blue text when not cached, but underlined purple text when cached. When the user activates the link (e.g. by clicking on it with the mouse) the browser will display the target of the link. If the target is not a html-file, depending on the file type and on the browser and its plug-ins, another program may be activated to open the file.
The HTML code contains some or all of the four main characteristics of a link:
link target (URL)
link class or link id
It uses the HTML element "a" with the attribute "href" and optionally also the attributes "title", and "class" or "id":
<a href="URL" title="link title" class="link class">link label</a>
When the cursor hovers over a link, depending on the browser and/or graphical user interface, some informative text about the link is shown:
It pops up, not in a regular window, but in a special hover box, which disappears when the cursor is moved away (sometimes it disappears anyway after a few seconds, and reappears when the cursor is moved away and back). IE and Mozilla Firefox show the title, Opera also shows the URL.
In addition, the URL may be shown in the status bar. Opera and Mozilla Firefox give the full URL, IE gives it from the last slash, or if it ends with a slash, from the last but one.
Hyperlinks as the currency of the World Wide Web
The Google search engine uses PageRank, a measure of link popularity to determine which page should be ranked first. The more pages that have a hyperlink pointing to a page, the higher rank that page gets. It is actually slightly more complicated than that, see PageRank for more information.
History of the hyperlink
The term "hyperlink" was coined in 1965 (or possibly 1964) by Theodor Nelson at the start of Project Xanadu. Nelson had been inspired by "As We May Think," a popular essay by Vannevar Bush. In the essay, Bush described a microfilm-based machine in which one could link any two pages of information into a "trail" of related information, and then scroll back and forth among pages in a trail as if they were on a single microfilm reel. The closest contemporary analogy would be to build a list of bookmarks to topically related Web pages and then allow the user to scroll forward and backward through the list.
In a series of books and articles published from 1964 through 1980, Nelson transposed Bush's concept of automated cross-referencing into the computer context, made it applicable to specific text strings rather than whole pages, generalized it from a local desk-sized machine to a theoretical worldwide computer network, and advocated the creation of such a network. Meanwhile, working independently, a team led by Douglas Engelbart (with Jeff Rulifson as chief programmer) was the first to implement the hyperlink concept for scrolling within a single document (1966), and soon after for connecting between paragraphs within separate documents (1968). See NLS.
Legal issues concerning hyperlinks
While hyperlinking among pages of Internet content has long been considered an intrinsic feature of the Internet medium, some websites have claimed that linking to them is not allowed without permission, see e.g.  (http://www.litmanlaw.com/content.aspx?page=243§ion=12) and  (http://www.stib.irisnet.be/msgN.htm) (in Dutch). You do not need to ask permission to link to any page of Wikipedia's.
In some jurisdictions it is or was (for example the Netherlands, see Karin Spaink) held that hyperlinks are not merely references or citations, but are devices for copying web pages. Although this principle is generally rejected by digerati  (http://www.edge.org/digerati/), the courts that adhere to it see the mere publication of a hyperlink that connects to illegal material to be an illegal act in itself, regardless of whether referencing illegal material is illegal.
British Telecom sued Prodigy claiming that Prodigy infringed its patent ( U.S. Patent 4,873,662 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4,873,662.WKU.&OS=PN/4,873,662&RS=PN/4,873,662)) on web hyperlinks. However, after costly litigation, a court found for Prodigy, ruling that British Telecom's patent did not actually cover web hyperlinks.