In this case, nothing. A TENS unit is an electrical power device that sends an electric current into the muscle with the use of conductive bi-polar sticky pads. The Kix e-stim stimulator is also an electrical power device that sends an electric current into the muscle with the use of conductive bi-polar sticky pads. The only difference here is that most companies the sell power units for electro sex use want to sound fancy and risqué, and that their product is special and unique, not like something you can pick up in the pharmacy section of Walmart.
Now all of that isn't to say all electro sex power units are just TENS units. Companies like ErosTek make literal monsters of power units that seem like you need a degree in electrical engineering just to understand what all they can do.