Here's a quote from wikipedia:
In arithmetic, when the result of the division of two integers cannot be expressed with an integer quotient, the remainder is the amount "left over."
You can look up remainder details in your GMAT quant book or on this page:http://en.wikipedia.org/wiki/Remainder
In your example, 9 is a remainder because 9 is less than 12 and there is no integer quotient.
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