Capacitors are (mostly) dipole elements that can hold charges when submitted to a voltage.
These charges are then oppositely accumulated onto the components electrodes  and the will be released if the component is still connected to a closed circuit. 
This is why an electric circuit can still be dangerous after being turned off: you may be shocked while touching loaded capacitors.
The capacitance C expressed in Farad, gives a technical indication of the charges q that a given capacitor under a given voltage U can withhold.

 q = C.U   and C = E0.S/d wih Eo is the dielectric permittivity between the electrodes, d is the distance between the electrodes, and S is the electrodes surfaces facing each other.