CNC stands for computer numeric control, and refers to machines that are controlled by computers. Examples of CNC machines include:
Chemical etching utilizes photolithography to create a "template" on top of the metal. Then the chemicals remove the metal not covered by the photolithography mask.
CNC requires additional tooling that chemical etching doesn't require. CNC is more costly and makes fewer parts at a time. Chemical etching is more affordable and the most efficient way to make a large number of parts at a time.