PART 1
DC motor: Continuous rotation, very fast.
Servo motor: Turn 90° in either direction for a total of 180° movement. Have a position sensor inside for feedback.
Stepper motor: Precise positional control, move 1 step each time. No position sensor for feedback.
PART 2
Example#1
A DC Motor fan which works for a while and rest for a while. I think it can be used for cooling down the computer when it operates tasks which generate lots of heat.
Example#2
A water pump made with a 6V DC Motor. I made a installation out of it. One side of the pump is up-taking water and the other is dropping the water in the same container. I used 4 AA battery as external power for the water pump and a potentiometer to adjust the speed of the water pump.