Brief description: Classical economists believe that in the short run the aggregate supply curve will slope upwards, but in the long run will be vertical. Equilibrium will be where aggregate demand equals supply.
Detailed description: In the short run it is possible for the economy to expand beyond full employment, but this will cause the price level to rise and in the long run the economy will return automatically to full employment, but at a higher price level. This results in a vertical long run aggregate supply curve.