When operated, the dome of the Van de Graaff generator and anything in contact with it takes on a charge. This charge can be positive or negative depending on how the Van de Graaff generator is designed.
Since like charges repel, this can have some entertaining consequences in electrostatics demonstrations. Let's pile a few bowls made of aluminum foil on top of the bowl.
When we turn it on, all of the bowls will be charged the same as the dome. The electrostatic repulsion will force the bowls apart from each other, sending them flying one by one off the Van de Graaff generator. Having these charged bowls flying about the room can be very dangerous, so please don--*flips switch*
Well, it turns out that wasn't dangerous at all, fortunately.