Contrary to the case of polynomials, in which the bases of powers, such as x^5, are variables, the bases of powers such as 3^x are constant and the exponents are variables.
An example would be y = 2^x. Note that the base, 2, is a constant, and that the exponent, x, is a variable.
A more advanced example of an expo function would be y = 7*e^5. Here, the base of this exponential function is e (which has the approx. value 2.718); the exponent is 5 and the coefficient of the exponential function is 7.