CNC Turning is the process of taking solid pieces of metal and slowly removing material to obtain a desired shape and or size. The material is removed by applying pressure with a cutting cutting tool while the metal block is turned thus the term "turning".
Because CNC Turning is a relatively slow process, it's better to use CNC turning to produce limited quanties or prototypes.
CNC milling differs from cnc turning in that with cnc milling, the part stays in a fixed position while the cutting bit moves around the part where as with cnc turning the cutting bit remains stationary while the part turns.