Fixed cost are expenses that do not change in proportion to the activity of a business, within the relevant period or scale of production. For example, a retailer must pay rent and utility bills irrespective of sales.
Variable costs by contrast change in relation to the activity of a business such as sales or production volume. In the example of the retailer, variable costs may primarily be composed of inventory (goods purchased for sale), and the cost of goods is therefore almost entirely variable.
Marginal cost is the change in total cost that arises when the quantity produced changes by one unit. In general terms, marginal cost at each level of production includes any additional costs required to produce the next unit. So, the marginal costs involved in making one more wooden table are the additional materials and labor cost incurred.