ActionScript and MXML relation

How do MXML and ActionScript relate to one another? The compiler, after parsing through the different idioms, translates MXML into the same objects, so that
< mx:button id="btn" height="100" label="My Button" >

and

var btn:Button = new Button();
btn.label = "My Button";
btn.height = 100;

produce the same object. ActionScript and MXML compiles into a SWF file