What is the USB output on a battery charger for?
The USB output is intended as a power source for devices that can be charged via a USB cable, such as smartphones, tablets or rechargeable flashlights.
Charging current: from battery to USB device
Battery chargers use batteries as power source. These batteries must be fully charged: If the battery charger contains no charged batteries, you also can not charge the USB device.
The picture below shows what it's like. The charging current from the mains is only needed to fully charge the batteries: Once charged it is possible to take the charger with batteries anywhere you want, so you can charge your USB device when it is needed.
What is important to know when purchasing a battery charger?
Voltage and amperage
USB is standard 5 volts and 500 milliamps (mA). This means that connected devices receive 5 Volt, where the charging current is 500mA. Remember though, that the charging current varies per source: different USB chargers deliver different charging currents.
Is it possible to overcharge my USB device?
No, it is not possible to get your USB device too much charging current. The USB device can arrange this for itself through internal soft-and hardware.
It may be that a charger can deliver more current than the device needs (for example by charging a smartphone with 1000mA, with a tablet-charger that passes on 2100mA). In this case, the smartphone uses only what it needs (1000mA), so there is no risk of overheating or other hazard.