From the official blog:
Since early March, we have been routing links within Direct Messages through our link service to detect, intercept, and prevent the spread of malware, phishing, and other dangers. Any link shared in a Direct Message has been wrapped with a twt.tl URL. Links reported to us as malicious are blacklisted, and we present users with a page that warns them of potentially malicious content if they click blacklisted links. We want users to have this benefit on all tweets.
When this is rolled out more broadly to users this summer, all links shared on Twitter.com or third-party apps will be wrapped with a t.co URL. A really long link such as http://www.amazon.com/Delivering-Happiness-Profits-Passion-Purpose/dp/0446563048 might be wrapped as http://t.co/DRo0trj for display on SMS, but it could be displayed to web or application users as amazon.com/Delivering- or as the whole URL or page title. Ultimately, we want to display links in a way that removes the obscurity of shortened link and lets you know where a link will take you.
In addition to a better user experience and increased safety, routing links through this service will eventually contribute to the metrics behind our Promoted Tweets platform and provide an important quality signal for our Resonance algorithm–the way we determine if a Tweet is relevant and interesting to users. We are also looking to provide services that make use of this data, an example would be analytics within our eventual commercial accounts service.
Already using your own URL shortener for analytics? Don’t worry – they’ve got that covered.
If you are already partial to a particular shortener when you tweet, you can continue to use it for link shortening and analytics as you normally would, and we’ll wrap the shortened links you submit.
(Source: Twitter blog.)