Route of the luxurious Air France Bus shuttle service network.
The vast Paris Metro system encompasses the numbered main lines, the lettered RER lines and the numbered tram lines.
Relaxing inside the ultra-luxurious Air France Bus 2. This bus goes straight to the Arc de Triomphe.
Roissy = Paris-Charles de Gaulle Airport (CDG)
When you arrive at Charles de Gaulle (also called Roissy) and you want to go straight to the Arc de Triomphe (known in transportation terms as Charles de Gaulle-Étoile, or simply Étoile), the easiest and quickest way is the [Air France Bus 2], the stop of which is right outside on the arrivals curb. Granted it ain't cheap, but for sheer speed, comfort and convenience it's well worth every euro. CDG is also the northeast terminus of the [RER B] train.
The colossal Arc de Triomphe is just one of the many photo-ops in this transit hub known as Etoile.
Charles de Gaulle-Étoile or Étoile = Arc de Triomphe, Champs-Elysées
The Arc forms the north end of the grand Avenue des Champs-Elysées as well as anchors the aforementioned Charles de Gaulle-Étoile subway transfer station. Lines serving this station include    and [RER A]. My mom and her tour group were staying at a hotel in the Issy-les-Moulineaux district out west. I didn't have a confirmed reservation anywhere, and their rate was pretty good that day, so I decided to book a room for myself there as well. To get to the hotel from Étoile, I took the  southeast to Champs-Elysées Clemenceau, then the  south to Invalides and then finally, the [RER C] west to Issy.
My mom and I inside a RER C train headed for the Eiffel Tower (Champs de Mars station).
Champs de Mars = Eiffel Tower (foot)
So I picked my mom up from our hotel and we took the [RER C] east to the Eiffel Tower (use Champ de Mars station). If you want to experience the craziness underneath the tower and/or would like to go up on it, this is the train to catch and the station to exit from. But you prefer your Eiffel Tower as a breathtaking view from a distance, then there are other vantage points from which to enjoy it.
Père Lachaise or Philippe Auguste = Jim Morrison's Grave
For a rock 'n roll fan, no trip to Paris is complete without a visit to the final resting place of The Doors frontman Jim Morrison. Also buried here are other music legends Chopin and Rossini, and author Oscar Wilde. The next day, I took Metro line  to the cemetery and paid my respects before continuing on to London via the Eurostar.
Jim Morrison grave, Pere Lachaise Cemetery. The Philippe Auguste station is closest to here.
Gare du Nord = Eurostar
You've heard about this amazing 2-hour bullet train between Paris and London that goes through a tunnel under the English Channel. Well in Paris, the Eurostar's terminus is Gare du Nord, served locally by [RER B] and many other lines.
Air France Bus 2:
CDG Airport (end) <> Porte Maillot <> Etoile (end)
La Defense <> Porte Maillot <> Etoile <> Champs-Elysees Clemeceau <> Musee du Louvre
Metro Line 2:
Etoile <> Blance (Moulin Rouge) <> Anvers (Sacre Coeur) <> Pere Lachaise <> Philippe Auguste
Metro Line 13:
Champs-Elysees Clemenceau <> Invalides
CDG Airport (end) <> Gare du Nord <> Notre-Dame <> Antony (Orly Airport)
Notre-Dame <> Musee d'Orsay <> Invalides <> Champs de Mars <> Issy <> Versailles