CNC stands for Computer Numerically Controlled, and refers specifically to the computer control of machine tools for the purpose of manufacturing complex parts repeatedly. Many types of tools can have a CNC variant:
The introduction of CNC machines radically changed the manufacturing industry. Curves were as easy to cut as straight lines, complex 3-D structures were relatively easy to produce, and the number of machining steps that required human action went way down.
In a production environment, all of these machines may be combined into one station to allow the continuous creation of a part involving several operations.
CNC machines are nowadays driven directly from files created by CAD packages, so an assembly or part can go from design to production without any intermediate drawing work being required.
CNC machines were relatively briefly preceded by NC, or numerically controlled, machines.