We normally use third party services to shorten URL before use it in Twitter tweet box prior to tweet. Otherwise we can’t fit a long link into the 140 character limit in tweets. Now you can paste a link (URL) of any length into the tweet box. Twitter will shorten it to 19 characters.
The main advantage of twitter link service is that once you posted tweet, it will show the shortened version of the original link, so people who see your tweet will know the site they are going to.
This is what posting a long URL now looks like:
Here is the shortened one
Twitter link shortening service is not available as a general link shortening service. You can only use it in tweet box. To protect users from harmful activity, Twitter uses t.co domain. Further to show the shortened form of original links (URLs), links shortened by Twitter’s link service are checked against a list of potentially dangerous sites and if found any match users will be warned before continue.
Shortened URLs from third party services are still allowable in tweets.
Images Via : Twitter