Offset lithography (printing) works on a very simple principle: ink and water don’t mix. Images (words and art) are put on plates, which are dampened first by water, then ink. The ink adheres to the image area, the water to the non-image area. Then the image is transferred to a rubber blanket, and from the rubber blanket to paper. That’s why the process is called “offset” — the image does not go directly to the paper from the plates as do other older processes such as letterpress or gravure.
If you look at your morning newpaper or a Sports Illustrated magazine, it is printed using this process. If it has color photos or type, it is printed using 4 “process” colors; cyan, magenta, yellow and black. Each color has its own plate and is printed on its own “unit” which can then have that color controlled to the amount of ink added to the plate. If you look really close at a picture in a magazine.