Under the tank heater is just a generic term to describe the location of the heat source. You can get an under tank heater made by different companies such as flexwatt, Zoo-med, Exo-Terra, Cobra etc. etc. They differ a bit in how they look, but all are a flat heat source, and they also need to be regulated by a thermostat. (or at least a dimmer)

And I'm with Kim, I need my flexwatt prewired for me.