Alternating current (AC) is electrical current that reverses its direction at a standard frequency of 60 Hz (cycles per second, or 50 Hz in South America and Europe). Conventional AC power is produced by rotating machines (alternators) that produce a smooth alternation, like that of a pendulum. It is described mathematically as a "sine wave". It is the ideal waveform for the transfer of AC power.
An inverter is an electronic device that converts DC to AC through a switching process. Thus it produces a sort of "synthesized" AC. There are two types of waveforms available from high-quality inverters. These are the so-called "modified sine wave" and the "true sine wave".
The "modified sine wave" is not really a sine wave at all. It is a stepped wave, like a pendulum that is being hit back and forth by soft hammers. It achieves voltage regulation by varying in width according to the battery voltage and the load. Thus, the wave is not as smooth as a sine wave. The quality of "mod sine" inverters should not be underestimated, however. They are highly capable, and (by narrowing the waveform) they save energy when running only small loads, as happens during most of the day in a typical home. They also cost half the price of sine wave inverters!
In spite of these occasional problems, mod-sine inverters have been successful in many thousands of remote home, RV and marine systems since 1986.
True sine wave inverters are more efficient for running motors, including AC pumps. They are less likely to draw complaints from people who enjoy high quality audio, or who simply have lots of electronic gadgets. If a mod-sine user has a problem with one or two small applications, here is a solution. Add a second inverter to the system, a small sine wave unit, to handle the problem circuits.