Why 28V DC bus is a de facto standard in Aviation industry?
To supplement Brian's answer:

It's partly historic of course. Based on sources of power available in the early days - such as lead-acid batteries and on the needs of components in electrical devices - such as radio receivers/transmitters

Car batteries are usually labelled as 12V. And they consist of six cells using a lead-acid chemistry. The cells actually produce closer to 2.1 volts per cell I believe.

The charging system however produces something closer to 14 V. So whilst the car is running, the electrical power is closer to 14 V. 

Aircraft basically have double this. I believe the batteries are 24 volt batteries and the charging system or electric generators produce 28 volts. These are described as 28V systems but car makers would perhaps have called them 24 volt systems.

I read that some aircraft (e.g. SAAB 340?) used 48 V starter motors, so two 24 V batteries were connected in series to produce this voltage. Note that when the engines are not running (and hence neither are generators presumably) you have 24/48 V from the batteries not 28/56 V.