Battery chargers are used to charge batteries from an external AC electricity source - typically a generator. Some sites may use shore power or grid power. Many inverters have built-in battery chargers which are generally the best value. If you are shopping for a new inverter and require battery charging as well you should consider buying an inverter with that function built in.
External chargers are used for systems that do not have battery charging built on, or require additional charging power. If you have a large battery bank and a large generator, you may find you can make use of additional battery chargers, reducing the running time of your generator.