Aquatint is the name of a technique of etching characterized by the use of rosin. A similar etching can also be made with spray paint or other substances, but traditionally, rosin extracted from trees is used.
The rosin chunks must be ground into a very fine and light powder, lifted by the slightest breeze. This is placed in a box where an air flow lifts it onto a subsequently inserted copper plate, where it settles, leaving a thin white whispy veil.
The plate is then placed on wires, and exposed to heat that melts the dust into tiny transparent droplets, looking like miniature raindrops on a window. When the plate cools, the droplets solidify, and resist to the etching liquid's action.
We then protect the parts of the plate that must be white with brush and varnish, and immerse the plate in etching liquid that will dissolve any unmasked (unprotected, bare) copper. We add varnish to cover the parts of the plate that must be light gray, then another bath, and so on, from the lightest to the darkest tones.
The artist defines the number of tones in relation to the number of immersions in the etching liquid. The durations of these immersions and the solution’s concentration determine the depth of the bite, which itself defines the amount of ink remaining on the plate after wiping, hence the tone of the color on paper after printing.

Below, a photo of lightly etched copper using aquatint. The black square is one millimeter.
An example of aquatint etching. We seldom use aquatint alone, but mix it with other techniques, in this instance the point. The mountain in background was etched using only aquatint. Click to enlarge.
Jean Cencig, Rivière boréale, 2006, eau-forte et aquatinte, 6" x 8"