Stepper motors will move only through a fixed angle. One step is the least count of a stepper motor. Stepper motors are used where accuracy is needed. Stepper moves only a fixed angle. Stepping angles available are 3.5,.75,1.5 degrees etc... There are two types of stepper motors

1. bipolar stepper motor

2. unipolar stepper motor

The disadvantage of a stepper motor is that it is having a less torque,even though the movement is accurate So if you want good torque and accurate movement we require servos . See the comparison in the following link

http://www.robotics.com/motors.html

http://tigoe.net/pcomp/motors.shtml

You can get stepper motor from an old floppy drive. But hard disk motor is not a stepper. More about floppy drive motor, you can find in this link

http://www.doc.ic.ac.uk/~ih/doc/stepper/flpystpr/flpystpr.txt

http://jewel.morgan.edu/~tmalone/dskdrv/dskdrv.html

I bought two stepper motors Rs.210 for each motors, look like the figure below. But mine was a unipolar stepper motor , 24V,600mA,3.5 degree,min  delay=4ms(i will explain in next page). But it was not able to give enough torque to drive a robot because of its less torque. For power supply i had to keep three 9V battery with a 7824 voltage regulator. That is too heavy for the motor to carry even though the robot was too small, consisting of one microcontroller PCB and a fiber glass base. My aim was to make a micromouse. So if you buy stepper motor buy motor of good torque ,small voltage and current ratings. I heard of 5V steppers available in India. So better go for a floppy drive motor or stepper motor available in printers for making robots