What is the difference between stepper and servo motors?

Stepper motors are a special type of motor, with more than 2 wires. Each set of wires is connected to a discrete coil, and manipulating the power to these coils allows the motor to move in discrete steps.

The common type of these motors, used in CNC machines, have 200 steps per revolution. That means that each time the motor takes a step, it moves 1.8 degrees around.

Due to the large number of steppers used in computer products randing from tape and floppy drives, to the biggest LASER printers, stepper motors are fairly cheap. Their control systems are also reasonably priced.

Disadvantage to steppers is that there is no feedback that the cutter head has actually moved. This is called an open loop system, and is more of a theoretical problem than a real one, but it does deserve to be mentioned.

Now, the servo motor. Usually built into the motor is an optical encoder, that lets the motor know whether or not the motion actually happened. In this way, feedback is generated, and the position of your cutter head is virtually assured. This feedback is sent back to the motor, and is called a closed loop system.

However, servo motors are very expensive compared to stepper motors, and also require a different expensive board to run them. Coupled with their greater speed, this makes servos the better solution.