What is a turbofan engine and how does it differ from a turbojet?

A turbofan engine has a large fan at the front that accelerates air both through the core (where fuel is burned) and around the core (bypass flow). The bypass air provides most of the thrust while moving at lower velocity than core exhaust, improving propulsive efficiency. A turbojet accelerates all air through the core, producing high-velocity exhaust suitable for supersonic flight but less efficient at subsonic speeds. Modern airliners use high-bypass turbofans (bypass ratio 8-12) for fuel efficiency, while military jets use low-bypass designs for performance.