There are two types of etching:

Hard ground
The action of acid working on the surface of the plate makes the incision which holds the ink. A wax ground is applied to the surface of the plate and this is removed with a needle (drawn). The resulting lines allow the acid to bite to varying depths, creating  sharp, light or darker, heavier lines.

Soft ground
The wax is much softer and is not removed with a needle. The most common method is to place a sheet of thin paper on top of the waxed plate and draw the image onto the paper. The ground is lifted from the plate, sticks to the paper and exposes the metal underneath. The line produced is very soft and is particularly suited to use with aquatint.