• Brainly User
They are used to  store energy or electricity to give a back up for an electric device stop instant fluctuation.. they are  usually are found in fans and electric motor 
lets take an example of a fan ....consider a fan rotating on it axis ,and then somehow you switch the button off..then also fan will rotate but now with lower speed and then suddenly it stops ...these rotation after switching off is due to capacitor..
Required charge to increase potential of any substance from 100v to 101v is known as capacity and a device which use for capacity is called capacitor. Q=cv