A Van de Graaff generator is a device for making very high voltage electricity. It usually has a metal sphere above a plastic tube. Inside the tube is a rubber band that is made to move by a motor.
A wire brush is attached to the metal sphere, and almost touches the rubber band. Another wire brush almost touches the rubber band at the bottom.
Rubber is an insulator, so electrons on it cannot easily move around. This means that when a few extra electrons are added, or a few are taken away, the negative or positive charge remains in that place on the rubber band.
The rubber is attached at the top to a pulley made of glass or Teflon, or some other material. As the rubber leaves the glass pulley, electrons are transferred from the glass to the rubber, because rubber holds electrons more tightly than glass does.
The glass is now positively charged. It pulls electrons from the wire brush at the top. This leaves the metal sphere with a positive charge. The positive charges repel one another, and flow to the outside of the sphere, leaving the inside uncharged, so it can always lose electrons.
As the rubber band moves around, the negatively charged rubber is moved down next to the bottom wire brush. The electrons jump onto air molecules and then onto the wire, always trying to get away from one another.
The rubber is now neutral, and is brought around to the glass pulley to start the whole process over again. The top sphere becomes more positively charged with each revolution of the rubber band. The electrons leave by the bottom wire, where they build up on any surface they are connected to. Eventually, the charge separation (voltage) is so high that electrons are stolen from the air, and the air becomes conductive. A big spark conducts electrons to the sphere, and the voltage drops again.
Then the whole process starts over.