ABOUT FOUR COLOR OFFSET LITHOGRAPHY
The first step in the process of lithography or offset printing is accomplished by photographically placing an image on a printing plate.
The plate is then mounted on the press where the image receives ink. The press does not transfer the inked image directly from the plate to the paper, but onto a rubber blanket which then offsets or transfers the image onto the paper. Single color jobs are printed on a single-color (one plate cylinder) press. Two color jobs can be run twice through a single-color press or once through a two-color press. Four color process printing can be run on a single-, two-or four color press. Process printing is primarily used to reproduce color photographs or artwork that contains shades and tones of colors. By using tiny dots of red, blue, yellow and black you can create all the colors and tones of the spectrum. The first step in process printing is to photographically separate the colors. Each separation negative is made by filtering out all color except the one desired.
explanation taken directly from: http://www.imperialprinting.com/litho.html