Inverters are any circuitry that converts a DC power supply into an AC power supply. There are numerous methods to achieve this most of which incorporate switching the DC supply alternatively across the load. Simple schemes usually only produce square waves but complex systems can infact produce high quality AC waveforms.
Inverters are often used when connecting DC energy generators such as wind and solar farms to power grids or in back up power supplies. Inverters are also useful in motor control where the DC supply is from a rectified AC.
There exist two main categories of inverters Voltage Source Inverters(VSI) and Current Source Inverters(CSI). VSI require a low impedance DC source where as CSI require a stiff, high impedance, DC source. No explanation really given in notes any suggestions why?