I've always thought hoarding was waiting until there was a shortage of something then buying up any you can find usually, but not necessiarially to resell at a later date for profit. Say for example you lived in LA and shortly after the hurricane hit you drove around to every store and bought up all the water you could find.
Preparing or stocking up is done well in advance of any need of whatever you are stocking up on, for example having several gallons of water stored in your home at all times, not just after a disaster.
Though the .gov's definition may not agree with mine.