The difference between rational map and polynomial map between affine varieties?

Could any body show me the difference between rational map and polynomial map between affine varieties? What are the relations between them? Please feel freely answering my question since I am a newbie to algebraic geometry and I am not talented.

Thank for reading my question.

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A rational map is one given by a quotient of polynomials, while a polynomial map is one given by a polynomial. –  Alex Becker Feb 16 '12 at 16:12
Thank Becker for that. I got a sensation about something deeper, but I could not find or say it concretely. –  Arsenaler Feb 16 '12 at 16:23
For an example, consider maps between the variety in affine 2-space defined by $x=0$ and that defined by $xy-1=0$. –  Alex Becker Feb 16 '12 at 17:40
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