The document discusses circular flow models which show the linkages between different sectors of the economy and how money flows between them. It describes the two sector model consisting of households and industry, the three sector model which adds government, and the four sector model that also includes financial and foreign sectors. The circular flow models illustrate how households supply factors of production like labor to businesses in exchange for factor incomes, and how businesses provide goods and services to households in exchange for spending. They can be used to understand national income calculation, trade policies, and monetary policy.