Final answer:
A linear cost function is a straight line, while a nonlinear cost function is not. Examples include a telephone line cost function and economies of scale in advertising.
Step-by-step explanation:
A linear cost function is a cost function where, within the relevant range, the graph of total cost versus the level of a single activity related to that cost is a straight line. An example of a linear cost function is a cost function for use of a telephone line where the terms are a fixed charge of $10,000 per year plus $2 per minute charge for phone use.
A nonlinear cost function is a cost function where, within the relevant range, the graph of total cost versus the level of a single activity related to that cost is not a straight line. Examples include economies of scale in advertising where an agency can double the number of advertisements for less than twice the costs, step-cost functions, and learning-curve based costs.
Therefore, the difference between a linear and a nonlinear cost function lies in the shape of the graph of total cost versus the level of activity.