CMYK = Cyan, Magenta, Yellow & Key (Black)
RGB = Red, Green & Blue
Why does K stand for Black?
The K in CMYK actually stands for ‘Key’. This comes from old offset printing methods. The “K” in CMYK is understood to be the key since in four-colour printing cyan, magenta, and yellow printing plates were aligned with the black plate which is the key. In other words the other colours are setup in relation to the black which is thus the "key" to the process.