CMYK refers to combinations or overprinted dots of four process colours: cyan, magenta, yellow, and black.
When combined, these four colours are used to create the wide range of colours that is discernible to the human eye.

This process is widely used for photographic prints or colourful designs where a specific ‘spot colour’ output is not required (such as a Pantone colour), as colour output via CMYK may vary from each print job.