Top: general layout of a turbofan engine. Note that a separate set of turbines drives the front fan, as in a turboprop. The term "high-bypass" means that most of the air in the exhaust comes from the fan and flows past the rest of the engine, rather than flowing through it. Bottom: Specific example: a British-built turbofan that powered the Lockheed L-1011 airliner. Note that the compressors are dual-spool.

Credits - NASA