Pieces designed for viewing on the Internet are delivered in a 72 DPI format in a RGB color mode. This is because computer monitors require about 72 DPI to give the illusion of full color depth and range. RGB stands for Red, Green and Blue. This is because CRT monitors create color via three electron guns (one for each color) that fire electrons at a phosphorous coated screen which, depending on its values, glow and produce color. Similarly, LCD monitors create color by sending an electrical current through liquid crystals on a substrate (a layer of glass) and depending on how they align produce different colors. This mimics the way the human eye interprets color. Our eyes have three receptors (called cones) that pick up the colors red and green along with light intensity. Then our brain uses this information to deduce the amount of blue light and the end result is our perception of color.
In printing we deliver the media in a 300 DPI format with a CMYK color mode. 300 dots per inch (DPI) has been discovered a long time ago to be the limits of human color perception, so not needing to waste ink we print using 300 DPI. Modern day printing, typically off-set lithography, utilizes a four color printing process. These colors are Cyan (C), Magenta (M), Yellow (Y) and Black (K). By adjusting these values we can print essentially any color imaginable. We are only able to perceive the visible light portion of the electromagnetic spectrum, except for Super-Man, who is able to see through objects using X-Ray vision.
Why are monitors RGB and printing CMYK? Monitors emit light and color in the same manner we perceive color, via red, green and blue values. Printed material, on the other hand, display color by what colors the printed material absorb and reflect back to the observer. Cyan, magenta, yellow and black server as filters by how they subtract varying values of red, green and blue from white light to give the perception of color. Is it complicated? Yes. Do we understand it? Absolutely.