They are the four most important sectors of the economy from a macroeconomic point of view.
External sector: The sector is committed to the export and import (external trade) of goods and services. If goods and services produced in this country are sold to the rest of the world, then that's an export. If goods and services are purchased from the rest of the world, then we are talking about imports.
Government: A state or government shall keep law and order, maintain growth and stability, and provide administrative services. The primary reason for a government is to undertake development projects like dams, roads, heavy industries that usually have long gestation periods. The government invests in education and the health sector and delivers these amenities at a insignificant cost. The purpose of any government is to serve, not to make a profit.
Firms: Enterprises are economic units that perform the output. They employ and organize production factors and undertake production processes to motivate the realization of profits.
Households: Households purchase goods and services for consumption and also provide factors of production such as land, labour, capital and entrepreneurship. Households supply the business's production market.