'M' stands for Magenta (a colour midway in the red/blue spectrum), 'Y' stands for Yellow and 'K' stands for Key (the black key plate).
As a rule CMYK is also the order in which a full colour image is printed.
Consistent high image quality.
Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
Quick and easy production of printing plates.
Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface.
Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.
Slightly inferior image quality compared to rotogravure or photogravure printing.
Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image/background areas when developed plates are not cared for properly.
Time and cost associated with producing plates and printing press setup. This makes smaller quantity printing jobs impractical. As a result, smaller printing jobs are now moving to digital offset machines.