The image on the right is a demonstration of different screen resolutions. The percentages listed next to each resolution represent how many visitors use each particular resolution.

Most web sites I design have a width of 740px, enabling them to fit within over 99% of visitors' monitors. Many times, a web site requires visitors to scroll down vertically, but it should fit within the width of their screen. If it does not fit, it will most likely have a negative effect on your visitor’s experience.