First and foremost milling is the manufacturing process whereby machines use shear tools/cutters to rotate and remove material out of a station called a work-piece at a certain direction.
CNC Milling on the other hand is a particular form of Computer Numerical Controlled machining whereby computer programs are used to calibrate and program the way the machine should work.
CNC milling machines have got to be the most popular out of milling machines simply because of its automation. These machines are differentiated according to the number of axes that the programs operate on. The more axes, the better the functionality. The axes, tagged by different letters like the X and Y axes, represent the horizontal direction on the work-piece (forward-and-back and side-to-side) on a flat station. The Z axis is for a vertical, up and down movement and the W axis is designated for a diagonal representation across a vertical area.
CNC machining centers nowadays use advanced 5-axis machines that operate on Computer Aided Manufacture programs for ultimate performance. These machines are helpful because they are able to produce any kind of shape and size that manual machines could not. Operating on angles of 180 degrees or higher is an additional advantage of the CNC Milling machines
The rigidity and strength of the work-piece and the cutting tool must be factored because of the intensity and precision of the process involved.