A capacitor is a device used to store energy as an electric charge, similar to a battery but they are able to release the charge much faster. The amount it's able to store is known as capacitance, this is measured in Farads (F). They comprise 2 metal plates (conductors) separated by an electrical insulator (dielectric).
There are many different types available with detailed specifications for their required function. The main elements are available in varying materials and these determine many properties of the capacitor. Some factors to consider are:
Capacitors are found all around us, they are used in most electronics from fans to hybrid electric cars - there are over 500 just in a typical smartphone. Here are some of their uses: