A "direct flight" means you stay on the same plane with the same flight number throughout your journey, but may include one or more stops along the way to pick up or drop off passengers, while a "non-stop flight" implies flying directly from origin to destination without any intermediate landings, essentially meaning the plane never touches down until reaching the final destination. These are often (incorrectly) used interchangeably, but a "direct flight" technically allows for stops, whereas a "non-stop flight" does not.