We often have to talk about colours when building a website or having something printed, but simply saying Cyan is not enough, one persons idea of Cyan could be different from an other persons idea of Cyan. For that reason we use various standard descriptions of colours so we both know we are looking at the same thing.
CMYK stands for Cyan, Magenta, Yellow and Key (black), these are the colours of the 4 inks often used in printing, When a printer selects a colour it has to decide on the quantity or each ink to use to achieve a particular colour. Cyan for example would be written as 100%, 0%, 0%, 0%.
For ease when discussing print colours we will often use PANTONE®, Pantone is a proprietary colour standard that gives each colour a number, here is Cyan is 16-4529 TPX. You can find out more about Pantone colours at pantone.com
RGB Stands for Red, Green, Blue, These are the colours of the pixels (individual dots) that make up the image on your monitor or TV screen. When choosing colours for a digital platform you have to decide how much of each colour you are going to use for each dot. Cyan for example would be written as 0, 255, 255.
When discussing RGB colours we often use HEX codes as a form of shorthand, here Cyan would be written as #ff0000.