Simply stated, 2 water heaters installed in Series means cold water enters the first heater, often referred to as upstream, and travels through the second heater, referred to as downstream, on it’s way to the demand point.  

Parallel water heaters (diagram) are fed supply water from a tee (equal incoming) and delivered to the demand source via a tee (equal outgoing). Parallel heater configurations deliver water equally.

Why the difference and which is best for me?  In situations where hot water volume requirements are frequently in high demand, parallel may be the best solution. Large volumes of water are heated and ready for the demand. Conversely, if hot water demand is only occassionally peaking, a series configuration would allow you to turn on/off the upstream heater at will, heating additional water when needed, then temporarily disabling the tank, thus saving energy, when the demand is not there. It’s important to mention, turning off the power source (not the supply) to the upstream heater will continue to give you hot water on demand from the downstream heater. However, vice versa would result in having to draw all the water from the downstream heater before the heated water at the upstream heater would be available to the demand point.

Another reason for installing a second water heater in series is you’ve already determined you want more hot water available, but you’re not ready to trash your current water heater. Adding another heater in series will allow you to use both heaters with doubled capacity. Usually a cheaper alternative than buying one water heater twice the size of the original.