How can I retrieve the control's message?

FROM : ???
DATE : Tue Jan 29 02:58:48 2008

I'm a Windows programmer, and have been written program for Linux some
times, now, I try to learn programming under Mac.

But, as you know, in Windows, there are many System-Defined custom messages,
such like WM_KEYDOWN, or, WM_NOTIFY, with these I can retrieve many
information or actions occurred on controls, such like the TreeView.

But in Mac, in cocoa, I can't find anything like them, I even don't know how
to hook the action when user input some text into the TextBox or when user
have select some node in TreeView, who can tell me, how to do this?

Thanks again.