There are a number of different types of capacitors. Many are used in electronics which do thing like phase shift to start an electric motor and blocking a DC voltage while "passing" an AC one. Current leads voltage with a capacitor by 90 degrees. Inductors (coils) are just the opposite.
The electrolytic capacitors is used in our case here as it has the capacity to store an electrical charge as Hotrod noted. They come with various values and voltages. I have read in old car manuals that by looking at the "spike" that forms on points you can tell if your cap value is wrong. The spike will form on one side or the other depending on the value.
OK, school is dismissed.

Six volt guy living in a twelve volt world