Charge from USB port and CAR adapter?

Question

I know some device (cell phone, mp3 player, etc) can be charged by car adapter or pc usb adapter. Is there any difference using those chargers other than direct charge from a charger? How does it work exactly?

Answer 1

The USB specification/standard specifies how the bus can be used to provide power to a device. There are strict requirements on the max. power a device can draw from the USB port.
If a device needs more power, it needs to use a power adaptor.