The GetForegroundWindow function returns a handle to the foreground window (the window with which the user is currently working). The system assigns a slightly higher priority to the thread that creates the foreground window than it does to other threads.

DECLARE INTEGER GetForegroundWindow IN user32

MSDN link:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/windowing/windows.asp

The GetActiveWindow function retrieves the window handle to the active window attached to the calling thread's message queue.