Backlinks

Backlinks (or back-links (UK)) are incoming links to a website or web page. The number of backlinks is an indication of the popularity or importance of that website or page. In basic link terminology, a backlink is any link received by a web node (web page, directory, website, or top level domain) from another web node (Björneborn and Ingwersen, 2004). Backlinks are also known as incoming links, inbound links, inlinks, and inward links.

Search engine rankings
Search engines often use the number of backlinks that a website has as one of the factors for determining that website's search engine ranking. Websites often employ various techniques (called search engine optimization) to increase the number of backlinks pointing to their website.

There are various factors for determining the quality of a back link. Main factor is the page rank of the web page giving the back link. Back link from high ranked site is of good quality. Second factor is the subject of the pages which are linked by a back link. If both sites are discussing the same topic, the back link is relevant and of good quality. Third important thing is the anchor text of the back link. If the anchor text is related to the theme of the website (where the link is pointing), then it is called a good quality back link. A good quality back link will increase the page rank of your website.


Obtaining backlinks from search engines
Most commercial search engines provide a mechanism to determine the number of backlinks they have recorded to a particular web page. For example, Google can be searched using link:wikipedia.org (or link:en.wikipedia.org) to find the number of pages on the Web pointing to http://wikipedia.org/.

Yahoo!’s Site Explorer is a method of obtaining the number of backlinks on a site.


Technical
When HTML was designed, there was no explicit mechanism in the design to keep track of backlinks in software, as this carried additional logistical and network overhead.

Some website software internally keeps track of backlinks. Examples of this include most wiki and CMS software.

Other mechanisms have been developed to track backlinks between disparate webpages controlled by organizations that aren't associated with each other. The most notable example of this is TrackBacks between blogs.

Source : http://wikipedia.org/