Mid Drives are found in many electric bikes and electric bike kits.  In a mid drive bike, a motor is added (somewhere about the middle of the bike) to drive (with an additional belt or chain) the pedal spindle.  The pedal spindle is also driven by the pedal chain ring (that's the big sprocket/gear attached to the pedal spindle that your pedal crank arms attach to).  So the mid drive motor drives your pedal spindle to assist your pedaling.
In a Swing Arm Drive the motor is mounted to the swing arm and a singular chain (or belt) then drives the rear wheel.  This style bike will have reduced maintenance and higher reliability than classic mid drive bikes.  The reason is the motor is not driving the rear wheel through all of the other components in a classic mid drive arrangement (creating high stress on components originally designed for foot power stress--not the higher stress caused by high power motors).  As the swing arm drive bikes from Denzel do not have a pedal option the mechanics are greatly simplified.
Hub drive bikes have the drive motor built into the hub of the front or rear wheel.  A hub motor is very simple in that it drives the wheel directly, thus no high stresses are added to any of the other drive components associated with (human powered) pedaling.  Currently, Denzel only offers the operable pedals on their hub drive bikes.