What is the difference between (PI)modified sine wave inverter and (PSW)pure sine wave inverter?

Modified Sine Wave Inverter: If your equipment can accept some voltage fluctuation, consider a modified sine wave inverter. These inverters provide mobile power at a more affordable price and come in a full range of sizes. 

Pure Sine Wave Inverter: If you want to run your equipment exactly to the manufacturer's specifications, choose a true sine wave inverter. With true sine wave, motor loads start easier and run cooler. Some equipment only operate properly with true sine wave power—laser printers, variable speed motors and digital clocks. Waveform is an important consideration when choosing an inverter. sine wave inverters provide premium power that is identical to or even better than power supplied by your utility company. Modified sine wave inverters provide good performance at a great value.