Electrical energy in a fighter aircraft
It's conceptually similar to an automotive electrical system. There is a generator (driven by the engine), batteries and some other stuff. 

Small planes tend to use 28VDC for the power bus, which is just double the voltage used in an automotive electrical system (voltage is quoted with the fan up front running, since there will be bigger problems should it stop- automotive systems are about 14V with the motor running). 

Some mid-size commercial planes that I'm familiar with have both 28VDC and "wild frequency" 120VAC which can be used for internal lighting and such like. The latter has a frequency that varies with engine speed but it's usually over 400Hz. High frequency is used to keep the weight of transformers and motors down, while still being low enough that conventional materials can be used. The use of 400Hz nominal in aircraft dates back to before WWII and the use of gyroscopes and such like. 

Military fighter jets have high power requirements for their size (eg. the F16 has a 40/60kVA generator driven by a hydraulic constant speed drive from the engine). Because of the high power consumption, higher voltages than 28V are used - 120/208 400Hz 3-phase- to keep the weight of wires down. The 1970-era F16 is a fly-by-wire so there is also a backup generator and multiple batteries. The 3-phase AC can be easily converted to 28VDC using a transformer-rectifier unit (TRU). 

You can find much more in this Naval Technician training manual.