What is the difference between the Classical and Keynesian models?
In the classical model, aggregate supply curve is vertical (price level on the y axis), meaning that output is fixed, constrained by technology and inputs. Prices are flexible. So that if the demand curve changes, the effect will be entirely on price level and not on output.

In the keynesian model, aggregate supply curve is horizontal at some price level. If demand changes, the effect will be entirely on output.

So the main difference lies on price flexibility and the power of increasing output through aggregate demand stimulus.