RGB stands for Red, Green, Blue and is an additive color model. What this means is that this color model is based on adding and mixing light – when you add Red, Green, and Blue light together, you create a pure white.
RGB is primarily used for designing elements that will be displayed on a television set, a computer monitor, a mobile phone, or any other kind of light source. Obviously, it is a device dependent color mode.
Anything that will be displayed on a light source or electronic device should be designed from start to finish with a RGB color mode.
Anything that could ever be used for print should be designed with a CMYK color mode.