I have a question related to stepper motors. What does the "phase" of a stepper motor mean? I mean when we say that a stepper motor is a 2 phase stepper motor or a 4 phase stepper motor, what does it really mean?
The better terms are '2-coil bipolar' versus '4-coil unipolar'. These two types of stepper motors both use 4 phases.
A phase (or step) is the momentary condition of the stepper motor: which coil(s) is (are) energised and in which direction.
For the 4-coil version each phase has current flowing through one of the 4 coils. One side of all coils is connected to the power, so 4 open-collector (or drain) transistors can be used to drive the stepper motor.
A 2-coil bipolar stepper has 4 phases too, but as it has only 2 coils it requires current through the coils in both directions. Hence it needs two H-bridge drivers.
The electronics for a 2-coil bipolar stepper is more complex, but IIRC gven the same amount of motor 'hardware' the 2-coil is more powerfull.
Have a look at this website. It explains stepper motors in detail and has some great videos to help with the concepts. http://www.pcbheaven.com/wikipages/How_Stepper_Motors_Work
best of luck!