Aggregate Supply is the total amount of goods and services in the economy available at all possible price levels. Aggregate Demand is the amount of goods and services in the economy that will be purchased at all possible price levels. In an economy, as the prices of most goods and services change, the price level changes and individuals and businesses change how much they buy. The aggregate supply curve on a graph illustrates the relationship between prices and output supplied whereas the aggregate demand curve shows relationship between price and real GDP demanded.
When aggregate supply (AS) curve and aggregate demand (AD) curves are put together, it shows the AS/AD equilibrium in the economy. The intersection of the AS and AD1curves indicated an equilibrium price level of P1 and an equilibrium real GDP of Q1. Any shift in aggregate supply or aggregate demand has an impact on the real GDP and the price level. 
Short-run macroeconomic equilibrium occurs when the quantity of GDP demanded equals the quantity supplied, which is where the AD and short-term AS (SAS) curves intersect. The price level adjusts to achieve equilibrium. Short-run equilibrium does not necessarily take place at full employment. Long-run macroeconomic equilibrium occurs when real GDP equals potential GDP so that the economy is on the long term AS curve (LAS) as shown in Fig 2.
The AS/AD framework illustrates the reaction of an economy to an increase in aggregate demand:
- In the short run, the AD curve shifts to the right and the equilibrium moves along the initial SAS curve. Real GDP increases and the price level rises.
- The money wage rate rises to reflect the higher prices, and the SAS curve shifts leftward, decreasing real GDP and further raising the price level.
- In the long run, the SAS curve shifts leftward enough so that real GDP returns to potential GDP. Further adjustments cease. Real GDP is at potential GDP, and the price level is permanently higher than before the increase in aggregate demand.
The AD/AS model also explains how the economy responds to a decrease in aggregate supply:
- The SAS curve shifts leftward, real GDP decreases and the price level rises. A period of time with combined recession and inflation is known as stagflation. 
Factors that Affect AS and AD
There are multiple activities that can cause shifts in the AS and AD curves. The following are factors that can shake the aggregate supply:
- The increase in nominal wages shifts AS to the left because costs of production increases, which lowers profits.
- The increase in prices of other inputs into manufacturing of products also shifts AS to the left because production costs increase. For example, the rise in the price of oil or electricity would increase costs for producers and lower their profits (so they produce less).
- The usage of technology can shift the AS to the right because it increases the productivity; as a result firms can produce more output with the same amount of resources (increases in efficiency). An example is computers.
Similarly, there are factors that can cause changes in the AD curve as well such as:
- When there is an increase in the country’s exchange rates, the net exports decrease and aggregate expenditure also takes a dip resulting in shifting the AD curve to the left.
- An increase in the income of the citizens will encourage them to spend more; eventually causing a rightward shift.
- Foreign income also has a significant impact on the aggregate demand. When foreign income increases, exports will increase causing the curve to shift to the right as a result of increased aggregate demand. 
The AD-AS framework divides the economy into two parts – the ‘demand side’ and the ‘supply side’ – and examines their interaction using accounting identities, equilibrium conditions and behavioral and institutional equations. The ‘demand side’ typically examines factors relating to the demand for goods and the demand and supply of assets. The ‘supply side’ typically examines factors relating to output and pricing decisions of producers, and factor markets. The framework ensures that neither demand nor supply side factors are overlooked in the analysis and that macroeconomic outcomes depend on the interaction between the different markets. 
This model scores highly in terms of simplicity. In terms of flexibility, it is comparable to IS-LM. It can straightforwardly be extended to deal with stochastic shocks and open-economy issues. However, where this model does fail badly is in terms of accuracy. Again the basic assumption concerning monetary policy is that the authorities fix the value of the money stock. This leads to the unattractive feature that the ‘equilibrium’ is one in which the price level has converged on a constant value.
There are other problems as well with the AD-AS model than the assumption of a fixed money stock. Colander (1995) pointed out that the model contains two contradictory accounts of aggregate supply. In deriving the aggregate demand curve a fixed price multiplier theory is assumed while in deriving the aggregate supply curve the underlying assumption is one in which supply expands to the point at which marginal cost equals marginal revenue. 
Effect of Monetary Policy
In the case of contractionary monetary policy, the money supply in the economy is decreased which further leads to a decrease in the nominal output, also known as the Gross Domestic Product (GDP). Additionally, the declined money supply in the market also leads to reduced spending by the consumers which thus shifts the aggregate demand curve to the right.
In the case of expansionary monetary policy, the central bank increases the money supply in the market by purchasing government bonds, and this pumps money into the market, and also decreases the interest rate as banks have more cash to loan to firms. Thus, firms begin to invest in order to increase output i.e. increased GDP. This leads to increase in employment. Additionally, as there is more money in the market, the consumer spending increases as well. All this activity shifts the aggregate demand curve to the left. 
Effect of Fiscal Policy
In pursuing expansionary fiscal policy, the government either increases spending, or reduces taxes or does a combination of both. As mentioned above, increase in the government spending shifts the AD curve to the right. Reduced taxes mean the consumer has more dispensable income at hand, and so can purchase more. This as well shifts the AD curve to the right. Plus, a combination of both increased government spending and reduced taxes also works in shifting the AD curve to the right. The extent of the shift in the AD curve due to government spending depends on the size of the spending multiplier, while the shift in the AD curve in response to tax cuts depends on the size of the tax multiplier.
The government uses a contractionary fiscal policy when there is a demand-pull inflation. It also facilitates in paying off unwanted debt. In the case of contractionary fiscal policy, the government either decreases spending, or raises taxes, or does a combination of the two. Less money rotation in the market leads to decline in the output which means reduced GDP. The consumer spending also takes a dip as there is lesser money available for expenses. Contractionary fiscal policy shifts the AD curve to the left. If the tax revenues exceed government spending then this type of policy leads to a budget surplus. 
- Economics: Principles in action by Arthur Sullivan & Steven M. Sheffrin – Pearson Prentice Hall – 2003
- Keynesian Theory and the AD-AS Framework: A Reconsideration by Amitava Krishna Dutt and Peter Skott – University of Massachusetts Amherst – Working paper 2005-11
- P Turner – International Review of Economics Education, 2006