Disabling the APN settings which connect your phone to your providers network means that your phone loses all internet connectivity until it is enabled again.
Saves battery when you're sleeping ot have no intentions of using the net for a while.
Juicedefender includes some functionality such as turning the APN on for a few minutes in intervals to let services sync with android or turning it on when the screen is lit.
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Latedroid: JuiceDefender