Some capacitor questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "bdsabds sadgsdgsda by way of Terry Fritz <twftesla-at-qwest-dot-net>" <fdgahbdfhbfdb-at-juno-dot-com>

What is the difference between pF uF, and the others? If there was just a
site that tells conversions, that is all I would need, since I know what
pico, micro and the lot mean. 
I see how capacitors do their job, but I don't understand how they are
charged by weaker power sources and how they get discharged in a circuit.
Also, what makes some capacitors discharge all their energy at once, or
slowly over time?
Thanks,
Jon
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