A turbofan is a type of jet engine, similar to a turbojet. It essentially consists of a ducted fan with a smaller diameter turbojet engine mounted behind it that powers the fan. Part of the airstream from the ducted fan passes through the turbojet where it is burnt to power the fan, but part, usually the majority, of the flow bypasses it, and doing this produces thrust more efficiently.
A few designs work slightly differently and have the fan blades as a radial extension of an aft mounted low pressure turbine unit.
All of the jet-engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient, and relatively quiet in operation. Turbofans are also used in many military jet aircraft.
