URL shortening is the translation of a long Uniform Resource Locator (URL) into an abbreviated alternative that redirects to the longer URL. The original URL shortening service was TinyURL, which was launched in 2002 by Kevin Gilbertson to make links on his unicyclist site easier to share. TinyURL remains popular today; other commonly used URL shorteners include bitly, goo.gl (Google) and and x.co (GoDaddy).
Short URLs are preferable for a number of reasons. Long URLs in text can make the accompanying message difficult to read and links can break if they fail to wrap properly. Although most email clients can now correctly handle long URLs, the use and popularity of shortening URLs has increased because of mobile messaging and social media websites, especially Twitter which has a 140-character constraint.
Although URL services often provide users with handy features such as the ability to customize short URLs and track traffic, some security analysts warn that the use of third party services is simply the addition of another attack vector. Many services are free and offer no service level agreement, which means the user must trust the service's ability to keep its servers secure.
Additionally, shortened links offer the user no clue as to where they lead and can be used to redirect users to infected content. To compensate, some services allow the user to add a special character at the end of the shortened URL. The addition of the special character allows the person to hover over the link and preview the page it is pointing to.
Reliability and availability are two more concerns. Even if a service guarantees 99% uptime, there will still be 3.5 days per year when its shortened links won't work. And as some users have found to their dismay, shortened links may no longer work if the service goes out of business.