Lyra 3S Nexus

Unlike a ‘traditional’ bicycle where you can see the gears on the outside of the back wheel, the Shimano Nexus has all of the gears safely tucked away inside a sort of ‘drum’.
Unlike a derailleur, you can shift gear when the bike is stationary – this is really useful if you have to stop at the lights halfway up a hill – no more setting off in too high a gear.
The Nexus hub is a fine piece of Japanese engineering, developed by Shimano over the past 20 years.