A hyperlink, also called simply "a link", is a reference in a hypertext document to another document or other resource. It is an integral part of the hypertext transfer protocol (http) for World Wide Web, but it is used also in offline documents, such as .pdf (portable document file, Adobe Acrobat native format) and in .XML (extended markup language). Hyperlink can be used to fetch content and save it, view it as a separate document or display as a part of the reference document.
The history of the hyperlink
The history of the hyperlink began in 1965. Theodore Nelson in "the Xanadu Project" transposed the idea from fictional microfilm cross-referencing system into the computer world. In a series of books and articles published from 1964 through 1980 the general concept was changed from linking whole microfilm pages to connecting specific lines of computer text. Primary concept was intended to use on single computer machine, however introduction of DARPA network boosted the idea into creating links between documents and files stored on several networked machines. The idea of connecting parts of a single document via hyperlink arose independently, but was quickly merged to the hyperlink system. Both concepts combined together were fundamental for creating World Wide Web.
How does a hyperlink work?
A hyperlink has two ends, called anchors, and a direction. The link starts at the source anchor and points to the destination anchor. However, the name hyperlink is often used for the source anchor, while the destination anchor is called the hyperlink target. Every browser shows text hyperlinks somewhat exposed (they usually mark it with a different color). Clicking on the hyperlink activates it and displays target document.
Hyperlink - measuring the Net
But hyperlinks are not only the way we surf the Net. Life on the Web without search engines is almost impossible today, because of unbelievable amount of networked information. Most search engines use so-called "page ranking" to measure which site may contain useful information. This mechanism is mostly based on hyperlink popularity. Although whole idea of "page rank" mechanism is more complicated, its general concept is based on a simple rule: the more pages have a hyperlink pointing to the ranked page, the higher rank that page gets. Of course, each hyperlink has different value, based on the popularity of the "source" site (This means simply that if your website is a target for hyperlink placed on the big site like CNet of Microsoft, it has much higher page rank than a site with several hyperlink connections from private sites). This mechanism is based on measuring of hyperlink's quality. Although not perfect, both mechanisms usually works well enough to determine which website has got good content and which hasn't.