What are the main differences between a derailleur and an internal hub when it comes to bicycle gear systems?

A derailleur is an external gear system that shifts the chain between different gears by moving the chain across multiple sprockets. An internal hub, on the other hand, contains all the gears inside the rear wheel hub, providing a cleaner look and protection from the elements. Derailleurs are typically lighter and offer a wider range of gears, while internal hubs are more durable and require less maintenance.