Inside the generator are two rollers connected by a belt. The motor turns the bottom roller, thus rotating the belt and upper roller. A metal brush drags across the bottom roller, and static electricity begins to build.
The rollers are always made from different materials. The bottom roller is constructed from material that holds a negative charge, such as silicon. The belt is made of a neutral material like rubber. The top roller is made from a material that holds a positive charge, like aluminum, and is surrounded by a hollow metal sphere. The difference in charge draws the electrons up the belt. When the electrons reach the upper belt, they meet another metal brush which transports them to the sphere.
The electrons gather around the surface of the sphere until it reaches a critical potential. The metal sphere must release some of the electrons. With a quick zap, the Van de Graaff generator releases a spark of electrons towards the metal of the discharge rod. As long as the generator is switched on the cycle will repeat. When the generator is turned off, and the stream of new electrons stops, the electrons around the sphere can stay put.