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Houston
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What does it do?

i understand that it stores energy and discharges it when filled completely, but why would you use it? what does it do for you in a circuit? what simple uses does it have?
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Silly-con Valley, Ca, U.S.
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Capacitors also take time to fill up and discharge. This can act like a cushion in your circuit.

A circuit without a cap is like slamming on the gas pedal of a drag racer. 0 - 100mph in a few seconds.

Add a Cap and resistor and it's like slowly stepping on the gas of a Cadilac. You'll get to 100mph eventually, it'll just take 15 or 20 seconds.

And just the opposite. No cap = slamming into a brick wall. With a cap = slowing from 100 to 0 over 30 seconds.

I'm sure others will have better examples and similes.
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10 PRINT CHR$(7)
20 GOTO 10

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The deep answer is that capacitors have state.

More practically capacitors allow filtering, store values as charge or voltage, decouple, couple, store energy, integrate a current...  There's so much they are used for its amazing!

Have a look in the learning section for circuits with capacitors - the commonest use is decoupling in a microcontroller, but in analog circuits everything uses capacitors.
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Manchester (England England)
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A capacitor is a frequency dependent resistor for AC.

Therefore it can be used to short out high frequencies and let low ones through if you put it across a signal, or the other way round if you put it in line with the signal.
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the commonest use is decoupling

True if you want to read about that then see:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
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SF Bay Area (USA)
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Capacitors do calculus!

Seriously.  This same question was asked about a month ago:

http://arduino.cc/forum/index.php/topic,52838.0.html

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Houston
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wow, thanks you guys... i guess i have to study up alot more than i thought! =)
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Manchester (England England)
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Just another point:-

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i understand that it stores energy and discharges it when filled completely,

No that's not how it works. It will only discharge when is is placed across a load or sees a voltage lower that it has charged to.
If it worked like you thought it would be an oscillator all by itself.
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