Etching is a method of making prints from a metal plate, usually copper or zinc, which has been bitten with acid.
The plate is first coated with an acid resistant substance (etching ground or varnish) through which the design is drawn with a sharp tool. The acid eats the plate through the exposed lines, the more time the plate being left in the acid, the coarser the lines. When the plate is inked and its surface rubbed clean, and it is covered with paper and passed under a cylindrical press, the ink captured in the lines is transferred to the paper.
The first etching on record was that of Swiss artisT Urs Graf, who printed from iron plates. Albrecht Durer, the famous engraver, made only 5 etchings, but Rembrandt really was the person who saw its potential and took it onto its next level, and it has subsequently been used by many legendry artists, notably, Pablo Picasso, Henri Matisse, Marc Chagall and George Rouault.
Today this is probably the most popular form of 'intaglio' printing.