An electrical motor powered by DC voltage. It uses two sets of wire coils to rotate a magnetic rotor by alternating the electric current between the two coils. These motors provide very precise rotary motion without the need for any sensors to provide feedback on the position of the motor.
The smallest motion possible by a motor is called a "step"--CNC tools keep track of their position by counting the number of steps that they have told the motors on each axis to make.
The number of steps that a motor must make to move by one inch (or millimeter in SI) is called its unit value.