Computer numerical control is a system in which a control microcomputer is an integral part of a machine or a piece of equipment (onboard computer).  The part programmes can be prepared at a remote site by programmer, and it may incorporate information obtained from drafting software packages and from machining simulations, in order to ensure that the part programme is bug free.  The machine operator can, however, easily and manually programme onboard computers.  The operator can be modify the programs directly, prepare programme for different parts, and store the programmes.
Because of the availability of small computers having a large memory, microprocessor(s), and programme-editing capabilities, CNC systems are widely used today.  The availability of low-cost programmable controllers also played a major role in the successful implementation of CNC in manufacturing plants.
Numerical Control is a system where machine action is created from the insertion of Numeric Data.  The Numeric Data is, in the beginning, written words in an easily understood code of letters and numbers (alphanumeric characters) known as a programme, which in turn is converted by the machine control unit (MCU) into the electrical signals used to control the machine movements.
NC Operation 
CNC stands for Computer Numerical Control.  An N.C. system in which a dedicated stored program computer is used to perform basic control functions.

The functions of a CNC Controller are:
1.To read and store programme information.
2.To interpret the information in a logical command sequence.
3.To control the motion of the machines mechanical members.
4. To monitor the status of the machine.

The interpretation of programme commands by a machine control unit and its conversion of those commands into machine motion is complex.  The basic elements and operation of a typical NC machine are shown in.  The functional elements in numerical control and the components involved follow:
a. Data input:  The numerical information is read and stored in the tape reader or in computer memor.
b. Data processing:  The programmes are read into the machine control unit for processing.
c. Data output:  This is information is translated into commands (typically pulsed commands) to the servomotor.The servomotor then moves the table (on which the work piece is mounted) to specific positions, through linear or rotary movements, by means of stepping motors, leadscrews, and other similar devices.