Gala is a window manager developed by the Elementary OS team. The developers claim that this window manager is more responsive and supports a series of attractive animations and effects. From the Elementary OS blog:
Nearly everything is animated in Gala, making for a more responsive user experience. While we have many rich animations, we are also careful to keep them modest and in most cases subtle. This allows us to benefit from the added responsiveness and polish while not slowing down the experience or distracting the user. Windows and popovers come in quickly, windows smoothly close, maximizing is slick, etc.
The developers have also released a video which you can view below:
You can read about the announcement here.