RGB stands for Red Green Blue, which are the three colors that the human eye can perceive. Every color in the visible spectrum can be made from these three colors. Televisions and computer monitors create images by combining Red, Green, and Blue.
The most common RGB input devices are CRT TVs and video cameras, as well as digital cameras and scanners.
Although video is displayed in RGB colorspace, as a combination of Red, Green, and Blue values, it's generally stored digitally in a compressed YUV colorspace of some kind. In order to ensure that the RGB image displayed is as close as possible to the original, standard Colorimetry systems are used to convert from one to the other.
The two most common Colorimetry standards are defined by ITU standards, based on systems developed in various countries. These standards are commonly referred to as Rec. 709 and Rec. 601, which are short for ITU-R BT.709 and ITU-R BT.601. In order for a signal to be reproduced correctly on a display, it needs to be decoded with a device using the same colorimetry as the encoder. Most modern hardware and software is capable of performing both Rec. 709 and Rec. 601 Color conversions.