CMYK

CMYK stands for cyan, magenta, yellow and black, the four process inks used to print full-color work.

K stands for key, the black plate. CMYK cannot reproduce every color a screen can, especially bright greens, oranges and blues, so colors designed on screen may print duller. Convert or check files in CMYK before final approval.

Not sure what your job needs? Ask the people who print it.