How it works: Strangely, this big update has gone relatively unnoticed. But it will be noticed as soon as you put a DVD in-- the app has a whole new fullscreen interface, including a time slider, and what Apple is calling an "Image Bar" (those are artist's renditions, so hopefully it'll look a lot better than that), which will let you access your chapters while watching the movie. There's an AutoZoom feature to deal with letterboxing, and it will "float" above your other applications, making it easier to watch movies while you're supposed to be working. Finally, videophiles will be happy to know that Apple is promising better video quality with "Adaptive Video Analyzation technology." Sounds good to us.
Who will use it: Anyone who likes movies is the obvious answer here, but I'm going to go a little more subtle-- anyone who likes movies and working on their Mac. So, you know, Roger Ebert. And the folks at Pixar.