t.co links on Twitter have been showing up for a while now, however the company has never implemented automatic link shortening, at least they hadn’t before today.
According to Twitter:
Just paste a link of any length into the Tweet box on Twitter.com. After you’ve composed your Tweet and you hit the “Tweet” button, we’ll shorten the link so that it only takes up 19 characters.
The program is meant to work with current security standards to ensure users are kept safe through proper Twitter system vetting for URLs.
The program as you may have noticed has also been cut down from the prior 20 characters to just 19, leaving users with a joyous 121 instead of 120 characters.
For users of other third party link shortening services such as Goo.gl or Bit.ly, Twitter still allows those links to work outside of their own service.