URL Shortening is one of the most effective tools for sharing useful websites is the URL shortening tool offered by a variety of sites.
Given that most social networking sites have limitations on the number of characters you can post in status updates and news feeds, it can be difficult to share links to websites that that have excessively long URL's. For instance, if you wanted to share a story on Twitter's addition of Hovercards as a great new feature, you'd copy and paste the following URL into your post: http://thenextweb.com/apps/2010/02/04/twitter-introduces-hovercards-quicker-check-someones-profile/. Then your friends and followers can click on that link and read the article for themselves.
But if you're posting that link on Twitter, and wanted to add your own thoughts on the story, you'd be limited to only 41 characters of additional text (the original URL above has 99 characters - Twitter's limit is 140 characters). But if you use a service such as TinyURL, you can shorten that 99 character URL down to 26 characters very simply, yielding http://tinyurl.com/yef9any - a link that will remain active as long as the original article remains at its original URL. With a web shortening service such as TinyURL, you just gained an additional 73 characters. Which can mean an awful lot when you're trying to get your message across.
statistics and definitions provided partly by wikipedia.org