Regression is the process of finding the line of best fit[1]. Interpolation is the process of using the line of best fit to estimate the value of one variable from the value of another [1], provided that the value you are using is within the range of your data. If it's outside the range, then you would be using Extrapolation[1].
[1] http://mathhelpforum.com/advanced-applied-math/182558-interpolation-vs-regression.html