Although the operating system can display windows from many different applications simultaneously during a PM session, the user can interact with only one application at a time-the active application. The other applications continue to run, but they cannot receive user input until they become active.

To enable the user to easily identify the active application, the system activates all frames in the tree between HWND_DESKTOP and the window with input focus. That is, the system positions the active frame window above all other top-level windows on the screen. If the active window is a standard frame window, the window's title bar and sizing border are highlighted.

The user can control which application is active by clicking on a window or by pressing the Alt+Tab or Alt+Esc key combinations. An application can set the active frame window by calling WinSetActiveWindow; it also can obtain the handle of the active frame window by using WinQueryActiveWindow.

When one window is deactivated and another activated, the system sends a WM_ACTIVATE message, first to the window being deactivated, then to the window being activated. The fActive parameter of the WM_ACTIVATE message is set to FALSE for the window being deactivated and set to TRUE for the window being activated. An application can use this message to track the activation state of a client window.