A Billet Ile-de-France is a small coupon like train ticket for Paris and surrounding Ile-de-France region priced for the length of the trip, calculated from departure station to arrival station. For example, if you were to buy a train ticket from Charles de Gaulle Airport to Paris, you’d buy a billet Ile-de-France, at a cost of 10€ for adults and 7€ for children 10 & under (as of January 2015). Books of ten RER tickets (a carnet) are discounted by 20%: 80€ for 10 adult tickets. 56.20€ for 10 child tickets.
These tickets are purchased from SNCF/Transilien/RATP ticket windows and ticket vending machines (Billetterie Ile-de-France) at large train stations (gares such as Gare de Lyon, Gare du Nord), and at RER and Metro stations.
At train stations there may be several types of ticket windows, one selling Billets Ile-de-France, and others selling tickets for the Grandes Lignes, tickets for TGV and other types of intercity trains. Grandes Lignes ticket windows cannot sell you billets Ile-de-France, so keep this in mind before queuing up to buy train tickets.
This is a Billets Ile-de-France ticket office.
This is a Grandes Lignes (TGV, etc.) ticket office.
When buying a billet at a ticket window you can usually just say the word “billet” [“bee-yay”] and the destination such as Paris [“Par-ee”], Aeroport Charles de Gaulle [“Ere-oh-porh Sharl-duh-gall”], Versailles [“vere-sigh”]. Often ticket sellers speak English, at least enough to understand where you’re going.
A one way ticket is known as an Aller-Simple [“Al-Eh somplh”]. Round trip ticket, which are simply two one way tickets, is an Aller-Retour [“Al-Eh Ray-tour”].
A billet Ile-de-France allows connections or transfers to be made onto Metro or RER trains when arriving at Paris. Transfers onto buses or trams however, are not allowed. For those modes of transportation, you’ll need a new Paris Ticket T+.