The difference between a Demi-John and a Carboy is size. Carboys are much bigger and hold more liquid than Demi-Johns.
Demi Johns and Carboys are designed as a secondary fermentation vessel.
The initial fermentation should be completed in a larger vessel such as a brewing bucket. This is because the initial fermentation is very vigorous and gives off a lot of carbon dioxide. The carbon dioxide helps prevent infection, but this can cause a large amount of foam on top of the fermenting liquid. If the initial fermentation is performed in a demi-john or a carboy, you risk the airlock becoming blocked by the foam, when can then lead to the possibility of an explosive eruption and a lot of cleaning up.
Once the initial fermentation slows, less carbon dioxide is given off. This means there is more chance of infection. This is the time to syphon your fermentation into a vessel with a smaller neck with an airlock such as a demi-john or a carboy.
The Demi-John or Carboy allows you to ferment wines over a long period and helps prevent infection from airborne bacteria.
Demi-Johns and Carboys are not designed to hold a carbonated liquid, so a solid bung for long term bulk ageing should only be fitted if you are sure the fermentation has completed. For security, a safety cork or airlock should be fitted.
