Total revenue is the total amount earned from sales and is calculated as price times quantity sold. Marginal revenue is the change in total revenue from selling one more unit. There are two types of costs - fixed costs that do not change with output like rent, and variable costs that do change like labor. Total cost is the sum of fixed and variable costs. Marginal cost is the change in total cost from producing one more unit. Businesses aim to produce at the quantity where marginal cost equals marginal revenue to maximize profit, which is total revenue minus total cost. E-commerce firms have lower fixed costs than traditional stores since they do not need physical retail space or large inventories.