When I reboot an EC2 instance, do I get the initial image again, or is the state of the hard disk before the reboot kept?
And what happens with billing, does the hour start again, or do I continue with the fraction of the hour I was in when I rebooted?
Rebooting an instance is like rebooting a PC. The hard disk isn't affected. You don't return to the image's original state, but the contents of the hard disks are those before the reboot.
Rebooting isn't associated with billing. Billing starts when you instantiate an image and stops when you terminate it. Rebooting in between hasn't any effect.
Rebooting keeps the disks intact.
If you shut down the instance and power up a new one, the disks will be reset to their initial states.
This doesn't apply to the EBS disks, which persist even across shutdowns.