Milling is a more specific process. It cuts workpiece by face milling(flat surface) and peripheral milling(deep slot, thread, and gear teeth).
Turning produces a workpiece as a cylinder. It is used in parts that need a symmetrical or cylindrical shape. Cutting tools are fixed while the workpiece is rotating. CNC turning is efficient in volume production, which improves process automation.
Drilling produces a round hole in the workpiece. The processing is performed by drill press or milling machine. According to the hole depth and precision, different drill bits or tools will be added for drilling.