difference between 2009 and 2012

what are the main difference 2009 and 2012?

You will not a get a straight answer to to this question. There are a lot of things that have changed.

Refer to the launch portal

http://www.microsoft.com/en-us/dynamics/products/ax-2012-launch.aspx

There are “Whats new” documents you can download. There is also a “What’s new for developers” document too.