This is the 99 cent Question.
The Conventional Generator has been around for a long time. It is basically a motor attached to an alternator. The motor is directly connected to the alternator and normally must run at 3600 rpm to produce 120 Volt 60 hertz AC power. If the RPM of the motor fluctuates so does the Hertz or cycles per second of the electrical output.
Even on a 10,000 watt Conventional generator when the draw is only 500 watts the motor still has to run at 3600 rpm.
The Inverter Generator is a relatively new invention. The Inverter Generator uses a different type of alternator which produces 3 phase AC power. This power is then converted to DC current. The DC Current is then converted (inverted) into 120 volt AC power. The resulting power is usually much Cleaner* than the power from most conventional generators.