What are the key differences between an internal gear hub and a derailleur system for bicycles?

An internal gear hub is a system where the gears are enclosed within the rear wheel hub, providing protection from dirt and damage. It allows for gear changes while stationary and requires less maintenance. On the other hand, a derailleur system has gears that are exposed and located on the outside of the bike frame. It offers a wider range of gear options but requires more frequent maintenance and is more susceptible to damage.