A CNC operator, as the name suggests operates a CNC (computer numerical control) machine, usually this includes loading and unloading workpieces, checking workpieces, setting program datums, setting up jigs and fixtures, replacing worn or broken tooling, cleaning swarf from the machine and basic machine maintenance.
Some operators also do programming although this is usually limited to altering a program to suit the tools being used; for example altering a program to give the correct drill depth after changing the drill.
The exact duties of a CNC operator varies from one machine shop to another.
A CNC programmer actually writes the program that runs the machine although this job is increasingly done by software.