[4][5] The distinction between primary and secondary groups serves to identify between two orders of social organization.
These groups are often long-lasting and marked by members' concern for one another, where the goal is actually the relationship themselves rather than achieving another purpose.
[4] The concept of the primary group was first introduced in 1909 by sociologist Charles Cooley, a member of the famed Chicago school of sociology, through a book titled Social Organization: A Study of the Larger Mind.
Since secondary groups are established to perform functions, individual roles are more interchangeable, thus members are able to leave and outgroup are able to join with relative ease.
Such groups can be understood to be ones in which individuals exchange explicit commodities (e.g. labour for wage, service for payment, etc.).
Examples include study groups, sports teams, schoolmates, attorney-client, doctor-patient, coworkers, etc.
[7] Categories are characterized by an aggregate of individuals who share something in common, but only become groups when their similarities have social implications.
[8] The degree of entitativity that a group has is influenced by whether a collection of individuals experience the same fate, display similarities, and are close in proximity.
More simply, as explained by Thompson and Hickey (2005), such groups are ones "that people refer to when evaluating their [own] qualities, circumstances, attitudes, values and behaviors.