Interpolation and extrapolation:
In most cases, it is desired to estimate values other than at the sampled data points. For illustration, we may want to estimate what the temperature was at 2:30 p.m., or at 1 p.m. Interpolation is estimating the values in between recorded data points. The Extrapolation is estimating outside the bounds of the recorded data. The one way to do this is to fit a curve to the data, and use this for the estimations. The Curve fitting is finding the curve which "best fits" the data.
The Simple curves are polynomials of various degrees. Therefore, curve fitting includes finding the best polynomials to fit the data-for illustration, for a quadratic polynomial in the form ax^{2} + bx + c, it means finding the values of a, b, and c which results the best fit. Finding the best straight line which goes through data would mean finding the values of a and b in the equation ax + b.