Monetarists believe that the economy is stable in the long run at the natural rate of unemployment, and the observed instability of the economy is caused by inappropriate monetary policy. Keynesians, on the other hand, believe that the economy is potentially unstable and observed instabilities are caused by fluctuations in AD and AS.
They believe that changes in the money supply directly changes AD, which directly changes GDP. They do not think investment is an important issue.
Monetarists also believe that without government interference, the economy would be very stable. The government caused the economy to become what it is today: downward wage inflexibility, business cycles, etc.