By definition flight is the process by which any object moves without support from any surface. The movement is made through an atmosphere or beyond it by generation of aerodynamic lift or aerostatically by the use of buoyancy or ballistic movement. Based on these lines, the journey by the way of an aircraft is also referred to as a flight.
The dynamics behind the airplanes (any aircraft) are the very same as stated above. There are a number of airlines which run various flights between multiple destinations in the world.
One can rightly say that the airlines and the flights they run have made the world a smaller place. The duration of a flight may vary depending on the distance between the destinations, the routes taken and other factors.
Having said so there is a list of top most longest flights in the world. The flight run by Singapore airlines from Newark to Singapore is the longest with duration being 18hrs 50mins. By same airline, the flight between Los Angeles and Singapore is second longest with duration of 18hrs 05mins.
The flight from Sydney to Dallas (by Qantas, 15hrs 25mins duration), from Johannesburg to Atlanta (Delta, 17hrs 05mins), from Dubai to Los Angeles (Emirates, 16hrs 30mis) and from Dallas to Brisbane (Qantas, 16hr) are few other longest flights.