RGB stands for red, green, and blue. This is the way the human eye see color and is also method used to create the images you see on a computer monitor. This is one of the reasons for the different calibration on a computer monitor than the color on a printed sheet. A printed sheet uses the 4-color process made up of Cyan, Magenta, Yellow, and Black inks, also known as CMYK.
Please see the CMYK Color Explained page in our Help section for a detailed explanation of the differences between CMYK and RGB color.