Dithering occurs when you try to display a colour on a monitor that is not capable of displaying that colour, resulting in a "speckled" appearance. A dithered colour is an approximation of a colour, that is made by combining pixels of two colours that are available. This happens most often on the web when webmasters use colours that fall outside the 216 colour web safe colour palette, and those colours are viewed by a person using a 8-bit (256 colour) monitor.