Fig.1: three ICs, four Mosfets and a handful of other components make up the controller. It can be use in "stand alone" or computer-controlled modes.
Stepper motors are everywhere. For example, every computer contains several (in the floppy and hard
disk drives - one popular hobbyist source). They're used because it is easy to
achieve very precise positional control - far better than you can achieve with a
"normal" motor (if you can control it at all).
Unlike a conventional motor, where you simply connect an
appropriate voltage and "away she spins", stepper motors require considerably
more effort to get them to work.