Gravity tank. Upon activation, water stored in the water closet tank is rapidly released into the bowl through openings in the rim. The rapid filling of the trapway triggers a siphonic action within the bowl and trapway.
Flushometer tank (pressure assist). A pressure vessel installed within the water-closet tank uses water-supply pressure to compress entrapped air. When the flushing mechanism is activated, the energy stored in the compressed air is released, causing a surge of water into a specially designed bowl.
Flushometer valve. A valve, external to the water closet and connected to the pressurized water-supply line, is designed so that when the flushing mechanism is activated, the water-supply line is opened to create a pressurized discharge into the water closet.
Electrohydraulic. An electrohydraulic water closet is a siphonic or washdown water closet that utilizes motors, pumps, and controllers to assist the flushing action by monitoring and controlling the flush and dictating the exact discharge from the tank into the bowl.
Vacuum assist. This is a modified gravity-tank water closet that incorporates a trapway in the bowl and tank components. Activating the flushing device creates a vacuum via an interconnection between the flush tank and trapway, which assists in waste removal.
Source: Plumbing Manufacturers Institute