The RGB color mode works best with electronics displays like LCD monitors, digital cameras, and scanners. The RGB model is referred to as an additive model, where colors are added together to produce what we see on the screen using the most basic colors red,blue, and green. When added colors the resulting color is lighter ultimately resulting in a white. Many pixels together composed of the three main colors red blue and green, to create an image on a digital, blending the colors together to make the desired one. The main purpose of RGB color modes are for digital use; mainly online, because the RGB mode was specifically made for the pixel format. It gives higher resolution on screen as an RGB than its CMYK counterpart. An example of when to use an RGB color mode would be when you Photoshop web icons which would really only be necessary for web viewing.
CMYK, if printers are using a digital printing method, they would print color on paper using CMYK colors. The CMYK is made up of four main color variants to create the desired color in order to create a variety of different colors. When colors are added together the results are in darker shades, when three of the four are added. The result is a darker brown color, and black when the key color is mixed in, the opposite of a RGB mode. It is ideal to use the CMYK standard when printing either a business card, stationary, or a newsletter. When using soft wares to make designs like a business card its best to use CMYK and 300 dpi for best quality when printing. More information on CMYK color variants can be found here.
There are many factors when creating a CMYK or RGB document in software’s like Photoshop, or Illustrator. Depending on what your client wants in general, you will need to use on or the other RGB OR CMYK color mode. Neither of the color modes are perfect, but good enough for the human eye. Again the output is dependent on the final product, if something is needed to be on screen or any digital works, RGB is the way to go. If the final product will need to be printed CMYK is the standard and best option because the colors wont be the same as if an RGB file would be printed.