My understanding is that there are now some very small heaters (intended for small tanks and/or people on a very tight budget) that are sold without thermostats (can someone confirm this please?) These would be identifiable because there would be no setpoint adjustment (temperature knob.) To me, this would seem pretty bad in that the thing would just stay on all the time, leaving the tank to just follow external temperatures up and down but just at a raised amount, carrying the possibility of overheating.
The traditional aquarium heater (whether internal or external) has a setpoint and thermostat, usually including a small lamp to let you know when it is actively heating. When the tank water temperature is about right, you turn the knob so that the light just barely turns on or off, meaning you want the heater to keep the tank close to this temperature. You then keep checking and adjusting this for a few days to be sure the setpoint is really doing what you want.
The main problem that beginners usually have is mistakenly thinking that the temperature scale numbers on the heater knobs mean something in the real world. Usually they don't. Instead, it is up to you "adjust" the number cap relative to the knob itself so that your setpoint actually matches what your real-world thermometer (a separate thermometer is needed in addition to the heater) is reading, usually across the tank away from the heater.
Sizing a heater to a tank correctly is not only about having enough power to raise the body of water to the correct fish environment given the household environment the tank is put in, but is also about not oversizing the power such that it could cook the fish if the thermostat became stuck in the on position. My understanding is that this happens less now than it did a few decades ago but it still happens occasionally.
Reasonable tank water circulation is another assumption of good aquarium heating of course. The warmed water must be circulated well enough, at least for some species of fish, that they don't spend all their time hanging out next to the heater element. The other side of that coin though is that obviously many natural situations are not heated evenly and many species may enjoy having different temperature spots to swim to (yet another of the hundreds of ways that we -don't- fully know or think through the creation of ideal artificial fish environments in most cases, perhaps.)
~~waterdrop~~
cant help you really mate, never heard of it myself 
if you cant find anything online, that would be a sign, at least for me, to look at a better documented alternative.
