As BOEINGBOY explained, a modern jet aircarft is with equiped engine/apu driven power generators that provide 115 Volts (V) AC 400 hertz (Hz) which can be converted to 28 Volts (V) DC for certain functions like charging the on-board batteries.

More interesting, is why does an Aircraft use 400 hertz (Hz) system when the standard is 60 hertz (Hz)?

The answer is weight, a 400 hertz (Hz) system uses less copper coils to generate power, thus, weighing less and less weight is every Aerospace Engineers dream.

Most any modern device that uses 115 Volts (V) AC 60 hertz (Hz) can operate on 115 Volts (V) AC 400 hertz (Hz) system, but the reverse is not true. If one were to simply plus in their 115 Volts (V) AC 400 hertz (Hz) device into an 115 Volts (V) AC 60 hertz (Hz) system smoke and fire would occur due to the metal inside the device overheating. There are ways around this by reducing the voltage but that is getting way off-topic.

Dan