Residuals

A residual is the difference between an actual response value, and that value as predicted by a regression line. (y minus y-hat)

Residual Plots
A scatterplot of the residuals vs the explanatory variable is called a residual plot. A residual plot is simply a means of showing the residuals in a clearer way than a normal scatterplot+regression line does. A line should be drawn across the residual plot at y = 0 to make it more clear what the values of the residuals are.

For least squares regression lines, the sum of all the residuals is 0.