You only have 140 characters to use in a tweet and you don't want to blow that on a lengthy url of some site you want to tell people about. So a lot of Twitterers use sites like bit.ly or tinyurl.com where you can make shorter links that redirect to that site. Yesterday, Twitter announced it's own url shortener that will automatically condense your url within the tweet so you have more of the 140 to play with and you don't have to go to a different site to get the shorter version. This keeps more visitors at Twitter and lets them analyze which sites their users are posting most, data mine that, learn it, maybe advertise with it. Unless I'm missing something, it completely dooms bit.ly.