Difference between an inverter generator and a generator?

An inverter generator typically has an engine that can be run at variable speed depending upon the load. The engine is connected to an efficient alternator whose output is converted to DC by a rectifier and smoothed somewhat by capacitors. This raw DC power is then inverted into smooth AC power of the proper frequency and voltage for the output (for example 110-120VAC 60HZ or 220-240VAC 50 or 60 HZ). Regulation is very good and the system will use less fuel if the load is small or variable since the control system will allow the engine to idle down when the load is small or throttle up when the load is large.

A standard "generator" or "genset" is an engine connected to an alternator and run at a speed that is some multiple of the desired AC frequency, regardless of the load on it (as the load increases the throttle opens more to keep the engine speed the same). The output of the alternator is connected directly to the load. Typical speeds are 1800 or 3600 rpm in the USA for 60HZ power when using a 4-pole or 2-pole alternator respectively. The engine speed control can be as simple as the mechanical feedback governor system used for any constant-speed engine.