What is the difference between RGB and CMYK...?
The screen on your computer displays colours in RGB (Red, Green, Blue) colours. This is what is called an additive colour model. As you add more of each colour, you get closer to white. With no colour, you get black.
Printed colours are exactly the opposite. They use the CMYK model (Cyan, Magenta, Yellow, Black) which is a subtractive palette. The more colour you add, the closer you get towards black.
Each of these colours models has a gamut or a range of colours that can be represented. There is some overlap within these palettes, but you would be surprised at how few colours are represented in both palettes. Thus the reason that the colours you see on screen don't match your printer. If the colour you want isn't one of the colours that is within the gamut of both models, it won't match. Well over half the colours won't match.
If that isn't confusing enough, let's throw in some other variables that further confuse things. The colours you see on screen are dependent on the video card and monitor that you use on your system. And as monitors age, the colours change. So two separate users won't even see the same colour on their respective screens unless their equipment is identical and the stars are in alignment. Then we throw in different brands of printers, different brands of ink and different kinds of paper. So many variables to consider and it all leads up to colours that just won't match.