According to the TechCrunch web site, this new feature will allow users to add 116 characters of additional commentary to the existing 140-character limit for a quoted tweet.
Previously,Twitter let users embedded quoted tweets by cutting-and-pasting its URL, but 'retweet and comment' lets you do that by just pressing a button.
Users, however, cannot embed the full re-tweet and comment.
The feature is now available on Twitter's site and iPhone app and will soon be available on its Android app.