If you're new to printing one of the first terms you're likely to come across is 'CMYK'. Here's why it's important you understand what CMYK is, what happens if you print in RGB, and why you can't just convert artwork from RGB to CMYK...
CMYK refers to the four colours professional printers use to print your artwork.
C = Cyan
M = Magenta
Y = Yellow
K = Key (black).
Each colour is layered up to create the final, full-colour image. Some printers call it 'four colour', and some 'process'.
RGB refers to the three primary colours digital screens use to create images on your display.
R = Red
G = Green
B = Blue.
With RGB, three colours are mixed together to create full colour images on screens.
The need for each is in their application - paper is white, screens are black.
Mix red, green and blue and you get white, but you can't make black... So with computer screens where the starting colour is black, you can create a vast array of colours by adding 3 primary colours together, along with the already black background.
With CMYK you create black with K (key - black) and by mixing in cyan, magenta and yellow (much softer colours) you can create almost any multitude of colours... except white.
This matters because it's not possible to print RGB colours on a CMYK printer - the printing software will attempt to interpret your artwork as CMYK if you do. Clearly, creating vivid colours with soft colours is going to be challenging, and your printing will end up looking faded and washed out - most noticeable on photographs where the eye has a good idea of what to expect, even without knowledge of the image.
You can create a CMYK colour space in all good design software, and this website will help you figure out colour values if you really have to convert from RGB to CMYK.
Your printer should be able to easily spot converted images and let you know there may be problems before they print.
Find out more at Total Printing's artwork guide.