To elaborate what everybody has said....
Traditional, non-CNC machines are controlled by human eyes based on measurements taken manually or on dials as machinists move the cutting tool or the bed with an object in various directions. Most machines have at least X, Y, and Z coordinates, so you have to accurately, and sometimes simultaneously move multiple dials. Obviously, this becomes very difficult with complex shapes, and sometimes impossible as some machines have as much as 12 axis.
CNC machines uses servo motor and computer to control the "dials," so that once "start" button is pressed, the machine takes over until the product is made (ie. sequence is complete) Also, there is a feed back from the movement back to the controller, so errors are minimized. With computer controlling the movement, all axis can be moved simultaneously, and complex shapes become possible. Also, repeatability is excellent.
The tools move in pre-determined paths. Old CNC machines used to use paper tapes with punched holes. An operator must program this into the machine before the cutting can begin. Modern machines use micro computers to do the same job.