Lithography (Source: International Paper Company)
Lithography: Greek for "writing on stone". Its general meaning is planographic printing, or printing from a plane surface. Lithography is more known by its common name "offset".
Surface chemistry makes the image area water repellent and ink receptive while the non-image areas become water receptive and (as long as water is present) ink repellent. The process is known as "offset" because the ink and watered plate print on a rubber blanket cylinder which in turn, off-sets this ink and water impression onto the paper which is held on a third (impression) cylinder.
The ink used is much like letterpress in that it is very viscous. The water used is actually known as "Fountain Solution" which contains special materials such as desensitizing gums, cleaners, buffering agents, etc. The plate is an aluminum base with a special oleophillic (grease loving) coating that becomes the image area. A delicate balance between ink and fountain solution must be achieved during printing.
Offset printing accounts for approximately 47% of all printing done today!