Macromedia explains that there are enough differences between ActionScript 1.0, which was used in Flash MX, and ActionScript 2.0, which is the standard in Flash MX 2004, to warrant the creation of an entire dictionary! This article introduces The ActionScript 2.0 Dictionary, published by Macromedia Press. The overview given here, and the chapter of the book that you can download, provides some insightful reading on what has changed in the language.