There are two primary types of gas water heaters: storage and tankless. Most water heaters are storage models. These are insulated tanks holding 150 to 2000 liters with gas burners. The storage tank stratifies with hot water at the top and cold incoming water at the bottom. 
The main feature of tankless water heaters is that they never run out of hot water—assuming the water heating capacity large enough to supply the needed hot water demands.
In this columns we’ll try to explain the differences between these two approaches and offer some guidance on choosing between them.