A pure sine waver inverter is more sophisticated and has a smooth voltage signal change over time, whereas a modified sine wave inverter has a choppy voltage signal change over time that resembles an ascending and descending stair-like plot of voltage over time. Older electronics such as cathode tube TV's, and simple lights, and pumps, can sometimes be supported by modified sine wave inverters. But even so, these devices may produce a humming noise or run hotter than they would with a pure sine wave.
Pure sine wave inverters will make these older and simpler devices run better, and devices or appliances such as digital clocks, fans, power tools, medical equipment, refrigerators, microwaves, and many fluorescent lights need to be used with pure sine wave inverters.