Lithography is a method for printing using a stone or metal plate with a completely smooth surface. It was invented in 1796 by Bavarian author Alois Senefelder as a low-cost method of publishing theatrical works. His method used an image drawn in wax or other oily substance applied to a stone plate as the medium to transfer ink to the paper. In modern times, the image is often made of polymer applied to a flexible aluminum plate. The flat surface of the plate or stone is slightly roughened, or etched, and divided into hydrophilic (water-loving) regions that accept a film of water and repel the greasy ink, and hydrophobic regions which repel water and accept ink. This process is different from gravure or intaglio printing where a plate is engraved, etched or stippled to make cavities to contain the printing ink, or woodblock printing and letterpress where ink is applied to the raised surfaces of letters or images.
In the early days of lithography, a smooth piece of limestone was used. After the oil-based image was put on the surface, a solution of gum arabic in water was applied, the gum sticking only to the non-oily surface. During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite. Chromolithography (color lithography) was invented in 1837; by using more than one stone, different colors can be added to the same picture. Each color needs a separate stone. The great posters of such artists as Alphonse Mucha and Toulouse-Lautrec are made like this. Complicated graphics may need twenty or more stones.
The Daily Omnivore 
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