Wiki

Tantalum capacitors.

A capacitor is a device which allows electricity to pile up by providing space for it to spread out. One of the simplest types just consists of two long strips of aluminium foil separated by a thin insulating strip of plastic, and then rolled up, with a wire connected to each strip. If you connect it to the two terminals of a battery, positive charge will flow out of the positive terminal of the battery and onto one of the strips. Since it's in close proximity to the other strip, it repels an equal amount of positive charge from that strip, which flows back into the negative terminal of the battery. If you disconnect the battery, the electrical charge remains until you connect the two wires together, allowing it to discharge.