Michael Sippey announced it on Twitter’s blog — think of it as the next stage of the animated GIF:
Like Tweets, the brevity of videos on Vine (6 seconds or less) inspires creativity. Now that you can easily capture motion and sound, we look forward to seeing what you create.
iPhone only, at the moment, and a separate app from Twitter itself. (More here.) An example:
I predict these will be embraced by those who believe the only thing GIFs were missing were rapid-fire jump cuts.
— Joshua Benton