Opportunity Cost: The value of the next best alternative that is given up when making a choice. For example, if you choose to go to a concert instead of studying for an exam, your opportunity cost is potentially getting a lower grade on the exam.
Marginal Benefit: The additional benefit gained from consuming one more unit of a good or service. For instance, eating one slice of pizza may give you satisfaction, but each additional slice may provide less marginal benefit.
Sunk Costs: Costs that have already been incurred and cannot be recovered. An example would be purchasing tickets for a concert but then deciding not to attend. The money spent on those tickets becomes sunk costs because it cannot be refunded.