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.

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