Photovoltaic modules produce direct current (DC) electricity. This is convenient if the goal is to charge a battery for energy storage in an off-grid system, but our electrical grid and most of our day-to-day electrical devices operate on alternating current (AC) electricity. Therefore, in order to convert the DC electricity into AC electricity, a device called an inverter is installed as a part of the photovoltaic system. In addition to inverting the direct current power into alternating current power, a grid-tied inverter also matches the phase and frequency of the inverted power from the photovoltaic system to the power on the grid, thus allowing the photovoltaic system to safely and effectively feed into the grid and to interact with the power already supplied to the facility on which it is installed.
The reverse process to inversion is rectification, and people in the United States actually deal with rectifiers everyday, largely without realizing it. Like photovoltaic modules, batteries operate using direct current electricity, and so every time you charge a battery-powered device, you need to supply it with a direct current. Therefore every power cord for a cellphone, laptop, iPod or any other battery-powered device has a rectifier built into it.