CNC Machine is Computer Numerical Control, while NC is numerical control.The user interface in both vary, in NC the input given is by punched cards or magnetic tapes. And the program cannot be edited immediately, to do the same the cards should be changed and the whole process was slower.In case of CNC, the interface is through the computer, hence any changes to the program can be made immediately through editing it on the spot. This adds to more convenience.
Firstly, NC is not a machining method, It is a concept in which a machine is controlled by programs.
These programs are written using machine codes usually numerical data, symbols or alphabets which are then fed into punch cards. Punch cards are basically storage device. They hold data in small punched holes. These holes are aligned in such a way that that can be read by computers or machines.
In NC method mostly the program is written using numerical codes so the name for the machine is given as numeric control machine. This method is widely used in various machining processes like milling, shaping,turning etc.
The program written gives proper description of the job, tool, cutting speed, feed rate etc required by the machine for the manufacturing operation.