CMYK - CMYK or four colour process is created using the primary colours of pigment: Cyan (blue), Magenta (red), Yellow and Black. Black is designated the letter K to avoid confusion with B for blue. Because the inks used are translucent they can be overprinted and combined in a variety of different proportions to produce a wide range of colours.
PMS - Spot colours, also known as PMS colours, and officially as Pantone Matching System colours are specific colour formulas that will reproduce accurately in print. Instead of simulating colours by combining primary colours (CMYK), spot (PMS) colours are pre-mixed with existing and published colour formulas.
RGB - If you are reading this, you are reading an RGB display via your computer. RGB stands for Red, Green and Blue. These are the three colours that create every other tone of colour that is visible on your screen. Whilst each monitor is capable of displaying a wide range of colours, there are still inconsistencies between computers. This is usually due to the fact that screens are not always accurately calibrated.