The document discusses reference groups, which are groups that individuals compare themselves to in forming values, attitudes, and behaviors. There are two main types: normative groups that influence broad values and behaviors, and comparative groups that influence specific attitudes and behaviors. Reference groups can be direct groups an individual interacts with, or indirect celebrities, leaders, or groups they do not interact with directly. The influence of reference groups depends on factors like conformity pressures, the group's expertise, and the conspicuousness of products. Celebrities, experts, average people, and company representatives are common types of endorsers used in reference group appeals.