@CiaraNi They're more common than you (and I, until we bought one weeks ago: was the "stock" option) realise.
It's basically a normal shifting / external gearing system with the mechanical cable from the handlebar to the gear replaced by a little motor inside the gearing thingy.
Small buttons on the handlebar do the shifting. Signal is send wireless or with a cable.
I assume eight years from now something breaks in the middle of winter and I'll be all pissed.