While an aggregate comprises merely a number of people, a group in sociology exhibits cohesiveness to a larger degree. Aspects that members in the group may share include interests, values, ethnic/linguistic background and kinship.
Primary groups consist of small groups with intimate, kin-based relationships: families, for example. They commonly last for years. The term was coined by Charles Horton Cooley.
Secondary groups, in contrast to primary groups, are large groups whose relationships are formal and institutional. Some of them may last for years but some may disband after a short lifetime.