The app is free! It's very popular right now, and easy to use.
The clips play automatically on a continuous loop. Users sometimes make stop motion videos, like the one I've embedded below. (I really like toast.) You can see how the video plays, and loops, automatically. Note that the video has sound but, by default, is muted:
Vine is one of the most popular apps available right now. Because it is easy to use, I recommend you try it out and join in on the fun!
Twitter takes aim at video, acquires video clip service, Venture Beat
How To Make Great Vines, Business Insider