Stepper motors have a large torque at low speeds. Even at 0 RPM, the motor can deliver torque, so it can be used as a handbrake (controller boards might get hot though). The rotor (the part that rotates) consists of a magnet with many poles. The stator (the standing part, that doesn’t rotate) contains coils to develop a magnetic field that helps the rotor to rotate in the desired direction.
The characteristic feature of this motor is the angular rotation per step, given in degrees. A stepper motor with an angular rotation of 1.8 degrees per pulse needs 360 / 1.8 = 200 steps to make a full rotation.
Unipolar vs Bipolar:
Unipolar: 6 leads, needs only 4 transistors (1 per wire, minus 2 common wires) to drive the motor.
Bipolar: 4 leads, driver needs an H-bridge to change polarity of the windings.
A typical driver for stepper motors is the ULN2003, containing 7 transistors, or the L297/L298, containing 2 H-bridges.
Halfstep and Microstep:
When a rotation of 1.8 degrees is not fine enough, a different motor can be chosen, or you can use a drive that supports halfstep or microstep.