A capacitor basically consists of two plates with an insulator in between, although in practice the 'plates' are normally rolled up in a can to save space. It can be used in a circuit to store charge for small periods of time.
Charge Q = CV where C is the capacitance in Farads
charge Q is measured in coulombs (C)
Energy stored, W = ½ QV = ½ CV2 joules
If the dielectric (the material between the plates) is a vacuum, Capacitance C = e0 (A / l) where A is the area of the capacitor plates, and l is the distance between them.
e0 is the permittivity of free space (8.85X10-12)
If the dielectric is another material, capacitance is given by:
C = ere0 (A / l) where er is the relative permittivity, which varies between materials.
Putting capacitors in series reduces the overall capacitance:
(1/C) = (1/C1) + (1/C2) + (1/C3) .....
Putting capacitors in parallel increases the total capacitance:
C = C1 + C2 + C3 .....
Note that the series and parallel capacitance formulae are the opposite of those for resistance.