Core groups are a type of organizational structure that consists of a small group of people who are responsible for the overall direction and performance of an organization. The two most common core groups are NUCLEI and cadres. NUCLEI are the innermost core of an organization, usually consisting of the most influential members. Cadres are the outermost core of an organization, typically consisting of the most experienced members. Both NUCLEI and cadres are essential to the success of an organization, as they provide guidance and support to the rest of the organization.