What is the difference between a modified sine and pure sine wave inverter?

Modified Square Wave: The most common,general-use inverters available are of the Modified Sine Wave variety, usually available at more moderate pricing compared to pure sine wave models. Modified Square Wave output inverters are designed to have somewhat better characteristics than Square Wave units, while still being relatively inexpensive. Although designed emulate a Pure Sine Wave output, Modified Square Wave inverters do not offer the same perfect electrical output. As such, a negative by-product of modified output units is electrical noise, which can prevent these inverters from properly powering certain loads. For example, many TVs and stereos use power supplies incapable of eliminating common mode noise. As a result, powering such equipment with a Modified Square Wave may cause a grain or small amount of snow on your video picture, or hum on your sound system. Likewise, most appliances with timing devices, light dimmers, battery chargers, and variable speed devices may not work well, or indeed, may not work at all.

Pure Sine Wave: Pure or True Sine Wave inverters provide electrical power similar to the utility power you receive from the outlets in your home or office, which is highly reliable and does not produce electrical noise interference associated with the other types of inverters. With its perfect sine wave output, the power produced by the inverter fully assures that your sensitive loads will be correctly powered, with no interference. Some appliances which are likely to require Pure Sine Wave include computers, digital clocks, battery chargers, light dimmers, variable speed motors, and audio/visual equipment. If your application is an important video presentation at work, opera on your expensive sound system, surveillance video, a telecommunications application, any calibrated measuring equipment, or any other sensitive load, you must use a Pure Sine Wave inverter.