In simple words, the rate of change of function is called as a derivative and differential is the actual change of function.

We can also define derivative in terms of differentials as the ratio of differentials of function by the differential of variable.

A derivative is the change in a function[math] (dy/dx)[/math]; a differential is the change in a variable[math] (dx)[/math]. A function is the relationship between two variables, so the derivative is always a ratio of differentials.