An agent has rational preferences if her preferences fulfill two properties, that they are complete and that they are also transitive.
Usually, goods are categorized into three different groups, which are: normal goods, inferior goods and Giffen goods.
Opportunity cost, to put it as simply as possible, is the price you have to pay in order to obtain something else.
One key element that affects how people behave in different situations is the information they have about the issue. But, generally speaking, people do not possess same levels of information. This difference in the amount of information between two parties is known as information asymmetry.