Unipolar vs Bipolar
Stepper motors can be distinguished in the way the polarity of the stator poles is changed: Unipolar stepper motors have stator coils with a center tap. Each part of a coil generates a magnetic field in only one direction. In contrast uses the bipolar structure the full coil:
Advantage of the unipolar structure is that only one transistor for one motor coil is needed. With the increasing integration of semiconductors, this advantage is unimportant nowadays. The main disadvantage is that only one half of the winding is energized, resulting in a lower torque or a worse ratio of size to torque.
In bipolar stepper motors, the entire winding is energized. In order to be able to reverse the current direction, two transistors are required at each end of the winding. Nowadays you mostly get stepper motors with bipolar design.