CMYK stands for Cyan, Magenta, Yellow and Black. They are called the Process Colours. The idea behind this is that you can duplicate almost all of the colour spectrum. Almost all printing that has colour, especially any that has full colour photographs, is using process colours. The images are made up of several dots, in varying densities, which, when placed so tightly together give the illusion of other colours. If you were to take any magazine and look at it through a magnifying glass, you would see several coloured dots, of Cyan, Magenta, Yellow and Black.
Spot colours are the opposite of CMYK. They are solid colours, not a mix of several others. When you look at spot colours on page, there will be no dots. It will be solid. The only exception is when a spot colour is laid down with a screen, which means trying to make it appear lighter by using dots to spread the ink coverage. The dots will however, still be only one colour.
RGB stands for Red, Green and Blue. They are another colour system used to match the entire colour spectrum as well. They are used most on electronic displays. The images you see on your monitor use RGB. That is how monitors, TVs, Phones, etc. interpret colour. It converts it to RGB. When RGB colours are used in printing, they are converted to CMYK for the presses. While this sounds useful and simple, there is often something lost in the translation between these two colour schemes. To get the most accurate colour, it is almost always recommended that images are converted to CMYK or RGB (if designing for electronics).
At Hignell, we do have an exception to this. Our iGen3 press has special software that converts RGB properly, so if your design is in RGB and is going to be printed on the iGen press, leave it that way. Our Client Service Reps will advise you on this when it comes time to submit your files.