External battery chargers come in all shapes and sizes. Some accommodate 2 batteries, some can hold 10. They can be cheap and simple, giving only 3 or 4 LEDs to show you how full the batteries are. Some are more expensive and therefore more advanced, providing digital screens to show the user exactly how much power is in the batteries, shown in voltage. They can also show you how long they have been on charge and how long is left until they are full.
Some chargers have an express charge function which charges 2 batteries in less than 2 hours. Normal charging would take anywhere between 2-4 hours to charge from 0% to 100%.