A capacitor is a device that stores electrical energy in an electric field. . A capacitor is created out of two metal plates and an insulating material called a dielectric. The metal plates are placed very close to each other, in parallel, but the dielectric sits between them. Electrical current is made up of electrons. When current flows electrons "flow" through the matierial. A capcator is hooked up soo the current (electrons) flow into one of the metal plates. They cannot pass through the dielectric material and as such "get stuck" on one side of the capacitor. The collection of negative charges (electrons) one one plate causes positve charges to collect on the other plate. 

As current flows the capacitor charges (electrons collect on one side). At some point the capacitor plates will be so full of charges that they just can't accept any more. There are enough negative charges on one plate that they can repel any others that try to join them. At this time the capacitor is fully charged. 

The charges in the capacitor will always try to find a path to one another. If circuit is changed (like by throwing a switch) so there is no more incoming current but there is a path between the two sides of the capacitor, the electrons (current) will flow discharing the capacitor. 

This experiment uses a voltage sensor to explore this charging and discharging of capacitors.