It's gas or electric powered geyser that fires up only when you're using the hot water tap. As soon as you open the hot water tap, a sensor in the geyser triggers the heating mechanism (can be gas fire or electric element) and starts heating the running water. It takes 10-15 seconds for the water to heat up fully for you. When you turn off the hot water tap, the geyser stops the gas / electricity.
The advantage with this is that you don't have gas running all the time, but the disadvantage is that it takes time to heatup and is generally not suited for more than 2 bathrooms.