To expand upon roenxi's answer: Bezier curves are an example of creating a cubic (degree 3) parametric polynomial curve by using four points to describe where you want it to go.

Curve fitting is about starting with a cloud of points, and trying to decide how to draw a polynomial (of whatever degree you want) that does a good job of describing the trend line of those points.

Then you can use that polynomial to approximately interpolate points between the samples, or extrapolate points beyond the samples as an example.