In etching the printing plate is covered in varnish. The design is scratched through with a needle. Acid is applied, which bites into the exposed metal, but does not affect the covered areas, thus etching the design onto the plate. The varnish is removed, and the plate inked for transfer to paper. The longer the acid is applied, the deeper the lines are etched, and the darker they are and faint lines are 'stopped out' with varnish after only a little biting. Lines can be cut directly into the metal with a needle, a process known as drypoint. The metal thrown up by the needle - the burr - catches the ink and creates a particularly rich effect.
The advantage of etching over engraving is that the lines can be made with something of the freedom of drawing.