The serve is the most complex motion in tennis by far.  These two videos are aimed at explaining the 2 main sources of power.  First, we start with the lower body and look at how to load the legs properly.  Second, we look at what I think it the most important power generator in the serve, internal rotation of the arm.  Everything that occurs with the legs is designed to maximize the speed of internal rotation.  It all the comes together at the point of contact and we look at how important the angle of the racket and arm are to transferring power into the ball.  These are elements all great servers have in common.