Cabinet Departments: These are major administrative units with responsibility for a broad area of government operations. Departmental status usually indicates a permanent national interest in a particular governmental function such as defense or agriculture.
Independent Regulatory Commissions: These are federal agencies created by an act of Congress that are independent of executive departments though they are considered part of the executive branch. They regulate some sector of economy with oversight from Congress.
Government Corporations: These are corporations within the public sector that operate on commercial basis while remaining under public ownership - think USPS!