The first Acid test was essentially to test browser compatibility for the box model. The Acid2 (try it out in the browser you’re on, Safari3 passes btw) test focused on HTML/CSS for static content.
Now Acid3 is being developed and will focus on the dynamic portion of the web, ie javascript. Finally, an Acid test I can get behind! The other Acid tests merely pointed out inconstancies in very edge cases of css and html rendering; not very useful. Acid3 ensures that javascript code written adheres to the ECMAScript spec, which is much more important in my mind.
John Resig has a good summary of the draft along with renders of the test in different browsers. (Many more here.)