Slightly longer version - When you stop a AWS instance, the instance is torn down completely and all local state/data (Ephemeral) on the hypervisor is lost. when you then start the instance again a new hypervisor with spare capacity is found and then booted with your chosen base disk image. Restarts do not cause your instance to torn down so you don't lose local state / data. If your instance uses network drives (EBS) then these drives keep state when your server is torn down, and are re-attached when the server is started back up again,