What's the difference (for the purpose) between positioning and ferry flight?
A positioning flight is a flight for the sole purpose of positioning the aircraft to conduct another flight from another airport. This is often done when the aircraft finishes its day in one city, but is needed in a different city the following day because another plane has broken down.
A positioning flight is technically a type of ferry flight, however the latter is a more broad term. Examples of ferry flights include delivery from the manufacturer, or flying to a different city for heavy maintenance.
Both flights are generally conducted without fare-paying passengers on board, but sometimes extra airline staff are carried.
Ferry flight refers to flying the aircraft from the factory or flying of the aircraft to or from major maintenance (overhaul).
Positioning flight refers to the aircraft to some place(airport) from which it is operated. For example, an aircraft may be positioned at an airport for normal operations from the next day.
Both are non revenue flights. They are often used interchangeably. NTSB, however, seems to consider ferry flights as a type of positioning flight.
Eurocontrol defines positioning flight as:
A non-revenue flight carried out to position an aircraft for a scheduled or non-scheduled flight or service.
There's no absolute accepted definition across the world, however generally it depends on whether passengers could be carried on the flight. In a positioning flight the airplane could carry passengers, in a ferry flight usually not due to the condition of the airplane (mechanical problems, extra fuel tanks) or the ownership.