The videos continuously loop when viewed and can be embedded with tweets anywhere. They also have sound. The Vine blog explains the service as:
“Posts on Vine are about abbreviation — the shortened form of something larger. 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.”
The Vine app is currently only available on iOS (no iPad version yet) for free. It will be available on other platforms soon. Users do not need a Twitter account to use the app although Twitter owns the service. But they can use their Twitter account to sign up.
You can check out some examples on what Twitter and Vine envision people doing with it on the Twitter blog.