Capacitors are one of the main ways technologies store energy. They have a specific advantage in some scenarios over batteries, which is the speed at which they release energy. While batteries release energy over a longer period of time -- sometimes for years, as is the case for watches -- capacitors quickly let go of a blast of energy. For instance, camera flashes need a big load of electricity all at once, so capacitors charge up just before that flash and then quickly let it all go when it's time to say "cheese."
Capacitors