A URL shortener service converts a long URL to a much shorter URL. The shortened version is much easier to include in a tweet, comment, or post. For example, http://blog.lauer.bz/2012/03/attack-of-url-shorteners.html would be cumbersome to include in a tweet, but the bit.ly shortened version http://bit.ly/z5DbQX is much more concise. Also, its easier to track the number of clicks and generate metrics by using a shortened URL.
