CMYK is used everywhere as way to achieve full colour printing. It's also known as process, 4-colour process and full colour process and uses 4 ink colours - Cyan (C), Magenta (M), Yellow (Y), and Black (K - which stands for Key). The colours are layed down on the paper as tiny dots which combine to create the illusion (to our eyes) of other colours. If you have a look at something that's been printed under a magnifying glass you should be able to see the 'matrix' of tiny dots of each colour!
There we have it - a quick intro to CMYK. Check out our website at http://ping.fm/994ZS if you're looking to get something printed - we specialise in Flyers, Business Cards and Leaflets. :)