http://www.ladyada.net/make/mshield/use.html
If you would like to have 2 seperate DC power supplies for the Arduino and motors. Plug in the supply for the Arduino into the DC jack, and connect the motor supply to the PWR_EXT block. Make sure the jumper is removed from the motor shield.
That's a quote from the page - replete with misspelling and sentence fragment.
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