Refer to figure.
Nowadays, commercial aircraft electrical system is supplied by two (or more) engine-driven generators regulated in speed by integrated drives. These generators’ characteristics are:

• 115/200 Volts, 3-phase AC.
• 400 Hz.
• 90 kVA.

28 V DC is obtained by converting the generators’ output AC via the transformer rectifier units. The APU generator is essentially the same as the main engine-driven generators, so it will provide exactly the same output. The APU will electrically supply the aircraft and other associated systems on the ground when the engines are not running, and the Ground Power unit (GPU) is not connected.
As to why nowadays commercial aircraft generators are integrated with a CSD in an IDGU (integrated drive generator unit) and use mainly 400 Hz AC is because a higher frequency allows the manufacturers to make smaller and lighter generators that would allow increasing the payload instead of carrying extra weight from the generators. Because the frequency of an AC generator mainly relies on the number of pairs of poles and the speed of the rotor, it is sufficient to increase the generator’s rotational speed in order to provide a higher frequency, and thus decrease the generator’s size appropriately.