A Simplified Explanation
These letters describe the primary colours used in printing. RGB stands for Red, Green and Blue, and in combination will produce almost any colour, as well as white.
CMYK stands for Cyan, Magenta, Yellow and Key colour (which is black), and in combination will produce many colour palettes as well as black. In this case white is the absence of any colour.
In simple terms, RGB is used on products which emit light, such as TVs, and CMYK is used on products which reflect light, such as paper, cloth, mugs etc.
Confusing? It gets worse.
RGB are the colours you see on your TV or monitor which is fine, but when it comes to printing it needs to be in CMYK, so a file used on the website will not be the same colour if printed in CMYK inks. Even when a file is designed in CMYK the printed product will look different to the proof on your screen if it is not calibrated properly.
In addition, there are some colours which do not translate well to CMYK. We have often been asked for fluoro greens, oranges and vivid cobalt blues in designs, but unfortunately they just do not print well. The technical term is “outside the CMYK gamut.” The only option is to avoid those colours, however spot colours can be used on envelopes with some types of printers.
Most photographs will print reasonably true to colour, with the exception of the orange and cobalt colours mentioned above, however the safest option is to change any printing file to CMYK before sending it to print in order to avoid any unpleasant surprises.
Pantone is a colour matching system
This may be printed on 5 or 6 colour printing presses. Swatches are available which show the relative colour values of CMYK for printing purposes. Some colours do not convert well, as they need 1 or 2 extra colours on top of the 4 colour process, as is shown in the photograph below. It shows Pantone colours and their CMYK conversions to the right.