A category is a collection of people who happen to share a particular characteristic
According to Henslin (2013, p.135-139), groups, otherwise known as social groups, are people who have something significant in common. Examples of groups may include family and a class of students. An aggregate is a collection of individuals who temporarily share the physical space but do not see themselves as belonging—examples of such maybe people in a restaurant or citizens riding the public service. A category is a collection of people who happen to share a particular characteristic. They have nothing else in stock and do not necessarily interact with each other. Examples of categories are people who have green eyes or people born in a particular state.
A primary group is a group in which one exchanges implicit items such as concern, love, caring, and support. Examples of primary groups are crisis support groups, church groups, and family groups. Secondary groups are large groups who have impersonal relationships and are goal-oriented; links are temporary. Examples of secondary groups include the athletic team, a group of co-workers, and university classes.
In-groups are groups in which individuals feel they belong or a group towards which one feels loyalty. Examples of in-groups, maybe people born in a specific region, may be compelled to support their region’s sports team or people with the same political affiliation. By contrast, our groups are social groups which an individual does not identify.
A reference group is a group that we refer to when we are evaluating ourselves. Examples of reference groups include neighbors, families, teachers, classmates, members of the church, mosque or synagogue, and co-workers.