About Twitter's URL Shortener (http://t.co)

Twitter created http://t.co as a URL Shortener in order to better protect users from malicious sites. A link created by Twitter's URL Shortener (for example, http://t.co/m5LlcjuEn5/) can represent a much longer link, and potentially harmful links can be re-routed to a warning page.

Twitter automatically shortens links sent out in Direct Message emails.

Twitter has begun testing http://t.co for links included in Direct Message notification emails. Our goal is to make Twitter a safer place by checking links included in Direct Messages to see if they match a database of potential web forgeries ("phishing" sites), sites that download malicious software onto your computer, and spam sites that request personal information.
If a link in a Direct Message is detected as potentially harmful, we'll display a warning screen when you click the link from your email. You should still always use your judgment when clicking links on Twitter or in email notifications, as this service will only protect against known or flagged threats.

You might not see http://t.co links in your notification emails.

Twitter is rolling out this feature to all users in stages. Once the feature is fully launched, email clients accepting HTML emails may display the long URL in your email, but clicking on the link should still redirect through the http://t.co shortener.   

Learn more about how URLs are flagged as potentially harmful.

If the URL of a site you manage has been flagged, please see this help page about flagged URLs for more information.

via help.twitter.com