“Although ActionScript 3.0 contains many classes and features that will be familiar to ActionScript programmers, ActionScript 3.0 is architecturally and conceptually different from previous versions of ActionScript. The enhancements in ActionScript 3.0 include new features of the core language and an improved Flash Player API that provides increased control of low-level objects.” (Adobe)
Core language features:
http://livedocs.adobe.com/flash/9.0/main/00000009.html
Flash Player API features:
http://livedocs.adobe.com/flash/9.0/main/00000010.html