The A4988 stepper driver is for Pololu stepper driver boards
and compatible clones. These boards use the Allegro a4988
stepper motor driver IC. (see Allegro website for datasheet)
This library diverges from others that are around, in that it
assumes that the MS1, MS2, and MS3 pins are connected to gpio
pins on the Arduino, allowing control over the microstepping
modes.
The A4988 is capable of microstepping down to 1/16 of a step,
enabling fine control over the stepper motor. This fine control
can be used in, among other things, 3D printers.