URL Shortening

URL shortening is a technique on the World Wide Web where a provider makes a web page available under a very short URL in addition to the original address. For example, shortening the long http://www.nytimes.com/2010/01/03/weekinreview/03carr.html to something like http://bit.ly/8INM6L.

Purposes

There are several reasons to use URL shortening:

Avoid URL garbling

Currently, web developers tend to pass descriptive attributes in the URL to represent data hierarchies, command structures, transaction paths and session information. This may result in a URL that is aesthetically unpleasant and difficult to remember. Copying a URL that is hundreds of characters long can make the URL garbled. Then a short URL is useful to copy on an e-mail message or a forum post.

Use the smallest space possible

On Twitter or instant message status even a 60 character long URL can be too long. A URL shortener can make short URLs such as http://hi.ly/1 (14 characters), http://tinyurl.com/gf65th (25 characters) or http://lnk.sk/acgz (18 characters) or http://tr.im/o65Tg (also 18 characters). In Twitter http://www can be replaced by www and 7 fewer characters can be used.

Manipulating visitors

URL shortening is a special kind of URL redirection, which is sometimes used in pranks, phishing, or affiliate hiding. For example tinyurl.com/ha56k0k redirects to the shock site goatse.cx in a prank. More recently,[when?] some of these services (br.st in particular) have started filtering all shortened links through services like Google Safe Browsing.

via Wikipedia.org