The app is the result of an acquisition Twitter made in October 2012 of a startup called Vine, which hadn't actually launched its app at the time it was bought. Users of the new app are encouraged to log in using their Twitter, although it's not a requirement.
In a separate blog post, Vine co-founder Dom Hofmann describes the aim of the new app, comparing its six-second maximum video length to Twitter's 140-character tweet restrictions:
"Posts on Vine are about abbreviation the shortened form of something larger," writes Hofmann. "They're little windows into the people, settings, ideas and objects that make up your life. They're quirky, and we think that's part of what makes them so special."
News of the app's imminent release leaked out yesterday when Twitter CEO Dick Costolo posted a Vine video of someone preparing a steak tartare, including a link to the official @vineapp account on Twitter:
As you can see, Vine clips automatically play when embedded in tweets, although their sound is turned off by default. The clips also play within Twitter's official mobile app. Users can add locations to their clips the app draws on Foursquare's places database for that with three options for sharing: Vine, Twitter and/or Facebook.
Why launch Vine as a standalone app rather than a feature within the official Twitter app? It's likely the same strategy as Facebook's Facebook Camera app: a separate app can be iterated more quickly to figure out what features work best, before integrating them back into the main app later.
The lack of an Android version at launch will draw criticism, however, given that Twitter would be expected to have the resources to handle the necessary porting and testing required for a simultaneous cross-platform launch.