A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall to prevent mixing or they may be in direct contact.
Instantaneous, or tankless, water heaters use high-powered burners to quickly heat water as it runs through a heat exchanger, eliminating the need for a storage tank.
A hot water storage tank (also hot water tank, thermal storage tank, hot water thermal storage unit, heat storage tank, hot water cylinder) is a water tank that is used for storing hot water for space heating or domestic use.