Clicking various windows activates them. The Canvas and Timeline activation commands and some of the commands in the drop-down menus across the top are gray and unavailable if the proper window is not activated. You can see which window is active by looking at the title bar at the top of each window. The activated window has a lighter gray color in its header than the other windows' headers.
A great way to activate any of the four major windows is to use the appropriate keyboard shortcut:
+1 activates the Viewer window.
+2 activates the Canvas window.
+3 activates the Timeline window.
+4 activates the Browser window.
What you do in each window can affect what happens in another. Figure 3.1 shows an alternative view of the interface, created by simply dragging the four windows around and resizing them accordingly .
Figure 3.1. The four major windows.
The first window we'll discuss in greater detail is the Browser window, which you can see by pressing +4.
+1 activates the Viewer window. 
