Actionscript 2.0 and 3.0

AS3 is a lot more different then AS2 because of the extensive use of OOP principles. You can still write AS3 on the timeline, but it's considered best practice to write you own classes. So learning all about packages, constructors, imports and inheritance is a must.

Also the event architecture has been rewritten from the ground up, but is a lot more easier and makes more sense. Might seem like your writing more code to begin with but it all comes together nicely.

A great book to read is "Foundation Actionscript 3.0 with Flash CS3 and Flex" and then you might wanna get "Foundation Actionscript 3 Animation Making things move" Both published by friends of ED.