From Twitter help
There are a variety of websites that can shorten your long links, making them easier to fit into Twitter's 140 character limit. These services can provide you with a shorter version of any URL you want to share. Then, you can use the shorter version in your Tweet, and anyone who clicks on it will be redirected to the longer URL. It's that easy!
Here are a few examples of link shortening services that are popular with Twitter users:
Answer by Liansmommie at 10:37 PM on Aug. 15, 2010
Answer by DarkFaery131 at 10:36 PM on Aug. 15, 2010