Not all of your visitors will use the same screen resolution, and even those who do may not have their browser window maximized all of the time. Even if they did, the fixed control bars that some of them have along one or two edges of the screen will affect the available screen area. As if all those factors didn't create enough variations, you also need to consider that different browsers use different amounts of screen space for browser menus and control bars (which may or may not be displayed).
To save you the effort of writing functions yourself to extract this information, I am supplying a set of six functions that will provide this information for you. Let's start by looking at these functions and what they return for your browser/screen resolution.