The input focus directs all subsequent keyboard input to this window. Any window that previously had the input focus loses it.
The SetFocus member function sends a WM_KILLFOCUS message to the window that loses the input focus and a WM_SETFOCUS message to the window that receives the input focus. It also activates either the window or its parent.
If the current window is active but does not have the focus (that is, no window has the focus), any key pressed will produce the messages WM_SYSCHAR, WM_SYSKEYDOWN, or WM_SYSKEYUP.