An inverter is an electronic device that takes DC (direct current) power from a battery and converts it to an AC (alternating current) form that emulates the power that we get from the grid. They are a convenient means of powering A.C. appliances from D.C. systems. Inverters are commonly available in the 200-watt to 5000-watt range. Modern mobile systems use only an inverter for A.C. power requirements. One of the biggest advantages of an inverter is that A.C. power is available on-demand without needing to start an engine.
Also see inverter chargers that combine a battery charger and inverter in a single product.