CNC stands for Computer Numerical Controlled. CNC Milling is simply milling that is controlled by a computer. The similarities to regular milling are:

• Block of solid material is bolted to a table*. The table and the block can translate on the xy axis.
• A spinning drill bit [spindle*] cuts material from the block to form the part

The differences are:

• CNC milling machine is controlled by a computer and follows a path
• Regular milling machine is controlled by a human operator
• The precision of a CNC machine can be as high as .0001 inch.
• CNC milling is much faster than a human can do
• A CNC part can be replicated perfectly many times

*see picture