Viewable area (VA) refers to the amount of a web page a user can see in their browser without scrolling up or down or to the side. This is important in web design since we want most important information to fit in the VA. For example, when a user enters the site through the home page, we generally want him/her to see the web site branding, all the important menu options, and the most important ads and information that will add value to their experience -- all without having to scroll, since many users will not scroll often or will ignore info outside of the VA.
Screen sizes are changing, especially in the US, due to the prevalence of wide-screens with strange ratios like 16:9 and 5:3 (typical monitors in the past have a ration of 4:3). This effects some user's VA since it makes the window much wider than it is tall. Likewise, high-resolution screens are making the VA bigger.
As of July 2002, the 1024x768 Extended Graphics Array became the most common screen resolution. This was an upgrade from the basic video dimensions of 800x600 which is still the most common format for international users. MH recommends designing sites that assume the basic VA of 950x650. This ensures the site will appear wide enough in 1024x768 not to appear too small, but will still be 90% viewable on older 800x600 setups. If a site is specifically for a international audience or older users, it is recommended that 800x600 still be used.
See Computer display standard wiki for more information about common screen sizes.