CMYK stands for Cyan, Magenta, Yellow and Key (black). These are the primary colours printers use to create the whole colour spectrum. This can also be referred to as full colour or four colour. It is important that any image within your print document are supplied as CMYK. Most photo/picture editing programs today have the editing capabilities to convert images to CMYK.
Here is an example of how CMYK works.
These colours are all 100% screen.