The update is available OTA or from Samsung's Kies software. Users who are rooted and don't want to wait can download the update from here and then manually flash it on their devices.
The Galaxy S III will receive Project Butter for a smoother, more fluid UI performance, the improved notifications, "freestyle widgets" that automatically reorder your homescreen content before they are placed and more.
Here's a video by Samsung showcasing the various improvements. Below it is a video dedicated to the improvements and novelties in TouchWiz itself.
Now with T-Mobile on the bandwagon it seems AT&T and Verizon are lagging behind, something their customers are very vocal about in their respectful forums.