How are capacitors specified?

Answer 1

Capacitance is measured in farads (F), after Michael Faraday. A 1 farad cap is really big; capacitors are usually measured in microfarads (µF), or thousandths of a farad. A power supply in a big power amplifier might have two caps of maybe 12,000 microfarads each

Answer 2

The capacitance rating is there because the energy stored in a capacitor is W = 1/2CV2, where W is the stored energy in joules, C is the capacitance rating in frards, and V is the voltage on the cap. The cap's capacitance is a necessary specification because circuit design and performance hinge on having caps the correct value for the configuration of the circuit. The voltage rating on the cap is the maximum voltage it can handle. If that rating is exceeded, the capacitor will arc internally, destroying the dielectric and most likely causing a very low resistance bridge across the cap. This bridge will rapidly discharge the cap, causing a high current internal discharge. If the energy stored is high enough, the cap will explode. Voltage ratings for caps are needed because for any given circuit, a capacitor must be selected that has a voltage rating sufficient to handle the applied voltages (plus a bit of "extra") for that circuit.