CMYK refers to standard full-colour printing using the 4-colour process. Colours are made up using overlayed tints of Cyan, Magenta, Yellow and blacK inks. This is the standard process used for most full-colour printing, eg magazines, calendars, posters etc.

Note 4-colour process does not mean you can only use 4 colours in your design!

See also: Pantone®, 4/0, 4/4