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's 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 pr… (More on Turbofan)