Some good answers.
Just to reinforce what has mostly been said: AC generators are more efficient, more controllable, and capable of higher output currents without the disadvantages of the high current switching one gets with a DC generator, and the consequent sparking which could be hazardous. AC is easy to convert to different voltages with simple transformers. High power devices such as heated windscreens, fans, electric hydraulic pumps and prop blade heaters can use 'raw' AC directly.
Most avionics and electronic devices however, use DC voltages between 5 and 28 volts in their circuits. If supplied with AC, they therefore need to convert this to a lower DC voltage first, which would need a transformer and smoothing power supply circuitry in each device. Much easier to supply a suitable DC voltage to such devices. Large power convertors called Transformer Rectifier Units, TRUs, are used to produce 28VDC from the 115VAC.
28V is the same as an aircraft battery voltage, so if the TRU derived 28 volts fails, you still have 28V from the batteries to keep the essential systems alive to get you back on the ground. Also, some devices such as radios and navigation lights will work from batteries when on the ground with no engines running.
This is why most aircraft have both AC and DC distribution systems.
Last edited by Uplinker; 6th May 2013 at 00:40.