Drive the bike. The hub gear.

Many bicycles are carried out with a hub gear or a gear derailleur.
With a derailleur gear it is clear to see how the operation is.
But what about a hub gear?
Because from the outside it is not clear clear what really happens on the inside.
And what are the advantages and disadvantages of a hub gear?

De Fietssite explains the hub gear..

For the English version, please visit HERE.