Réseau express régional d’Île-de-France

On a first trip to the French capital, many visitors find themselves a bit confused by the public transport network. They often arrive in Paris at the Gare du Nord station via the airport, on a train referred to as the “RER B”. This may lead them to assume that the train in question is part of the city’s main metro network when, in fact, it’s part of a separate, regional system.But what exactly is the difference between the metro and the RER—and why does this matter for visitors trying to get around the city in the most efficient way possible?