Computer displays have steadily increased the number of pixels and colors they can display over the years. In 1987 the IBM PS/2 could produce a resolution of 640 by 480 pixels. Today some PDA’s can display 640 x 480, HD televisions can have up to 1920 x 1080 and there are computer displays of even higher resolution. Even so these displays do not reproduce exactly what our eyes see. Part of the problem is that all of the pixels that make up a computer screen or television are rectangular.
Low resolution displays are weakest when reproducing curves or lines that do not run perfectly vertical or horizontal. Aliasing is what we call the jagged edges and artifacts that result. One method of reducing the effects of aliasing is decreasing the size of the pixel. A typical computer screen has about 72 pixels per inch (ppi). A display of 300 ppi would be capable of producing smoother images because at that resolution it difficult to distinguish Individual pixels.
Another method is anti-aliasing, the process of smoothing the jaggies by coloring adjacent pixels with an average of the two. As you can see here, black text that has been anti-aliased on a white background has pixels of various shades of gray along the curved portions to give a smoother appearance.