If you hold a metal-coated ball near a Van de Graaf generator, the ball will be attracted to the generator. That's because the electric field from the generator is diverging, getting weaker the farther you go from the globe. The dipole moment induced in the ball makes the positive closer to the negative generator globe, hence it is attracted. If you hook the generator to the plates of a parallel plate capacitor and hold the same ball between the plates, you see that the ball is not attracted to either plate. That's because the electric field is uniform between the plates (to an approximation).

However, if you touch the ball to one of the plates, it picks up the charge of that plate and is accelerated toward the other plate. F = q E Once it touches the other plate, it transfers its charge to that plate and picks up charge of the opposite sign --- the charge sign of this plate. It is now accelerated away from this plate toward the other, and so on. The effect is the ball bounces back and forth between plates. If you turn off the generator, the ball still bounces for some time because of the charge stored in the capacitor. Gradually, the ball slows down as the ball continually transfers charge from one plate to the other, reducing the "charge on the capacitor", and hence the electric field between the plates.

See also:
Capacitance - Capacitor Size
Capacitance - Variable Plate Display