Recursive relationships: relationships exist between entity instances of the same type.
One-to-one: If we were to track which employees were married to other employees, we would expect each to be married to either zero or one other employee at any one point in time.
One-to-many: It is common to track the employment “food chain” of who reports to whom. In most organizations, people have only one supervisor or manager. Therefore, we normally expect to see each employee reporting to zero or one other employee, and employees who are managers or supervisors to have one or more direct reports.