Keynesian economics argues that aggregate demand is the primary driving force in an economy. Keynes proposed that during recessions, private sector demand is often insufficient, necessitating active government intervention through fiscal and monetary policy to stabilize the economy and achieve full employment.

The Keynesian model is primarily focused on short-run fluctuations in output and employment, emphasizing sticky prices and wages. Conversely, the classical model focuses on the long-run equilibrium, assuming that prices and wages are fully flexible and that the economy naturally tends toward full employment over time.

In a basic two-sector economy consisting only of households and firms, aggregate demand is defined as the total planned expenditure on goods and services. This expenditure is comprised of consumption spending by households and investment spending by firms. Since there is no government or foreign sector, the total demand is simply the sum of consumption (C) and investment (I), representing the total output required to satisfy both private consumption and capital formation needs.

Cyclical unemployment is the standard term for job losses caused by a downturn in the business cycle and a drop in aggregate demand. The provided answer identifies 'Seasonal unemployment' as the correct choice, which is factually inconsistent with standard economic definitions. Seasonal unemployment is related to predictable, recurring changes in demand for specific industries, not a general decline in aggregate demand.

Cyclical unemployment arises from a deficiency in aggregate demand during economic downturns. Standard fiscal policy to combat this involves expansionary measures like tax cuts or increased government spending. Encouraging savings is generally considered contractionary as it reduces consumption. The provided answer is unconventional, as increasing savings during a recession may exacerbate the paradox of thrift, potentially worsening unemployment rather than resolving it.

Keynesian economics emphasizes that the level of employment is determined by the level of aggregate demand. When aggregate demand is insufficient, firms reduce production and lay off workers, leading to involuntary unemployment. Unlike classical models, Keynes argued that wages and prices are 'sticky' downwards, meaning they do not adjust quickly enough to restore full employment automatically, necessitating government intervention to boost demand.

Supply-side economics focuses on increasing the aggregate supply of goods and services. This is typically achieved through policies like tax cuts, deregulation, and investment in technology, which aim to improve productivity and efficiency, thereby fostering long-term economic growth without relying on demand-side stimulus.

According to Keynesian theory, government spending is a component of aggregate demand. An increase in government spending, holding other factors constant, shifts the aggregate demand curve to the right, which increases the equilibrium level of national income through the multiplier effect.

Structural unemployment occurs when there is a long-term mismatch between the skills workers possess and the skills demanded by employers. This is often caused by technological advancements, globalization, or shifts in consumer preferences that render certain industries obsolete. Unlike cyclical unemployment, which is temporary, structural unemployment is persistent and requires retraining or relocation to resolve.

The Keynesian revolution challenged the classical view that markets are self-correcting. John Maynard Keynes argued that the market is inherently unstable and prone to periods of involuntary unemployment and low aggregate demand. Consequently, he advocated for active government intervention through fiscal and monetary policy to stabilize the economy and achieve full employment, as the private sector alone cannot guarantee stability.