Travelling in Paris is no problem at all since there is a variety of transport available, suiting the mood and the budget
of the traveller.
Paris and its suburbs are divided into five travel zones linked by the Métro(underground railway), bus and the RER. Zones 1 and 2 cover the city centre. The Métro is the most convenient way of travelling around Paris. RATP, the Paris public transport system, controls the Métro and buses. The public transport also includes the RER, the fast suburban overground lines which interconnects with the underground railway system inside Paris. What more, the same tickets and passes are valid on the Métro, bus and RER.
Métro Ticket

A carnet of ten tickets costs about £6 while individual tickets cost more. You can buy a weekly Carte Orange travel pass (passport photo needed) which offer unlimited travel in the zones 1 and 2 for about £9. Children under 12 pay half price.
Métro
If you are in a hurry, the Métro is what you should hop into. Apart from
being convenient, métro ticket price is quite reasonable too. In fact, travelling in Paris by métro also costs less than most major cities in Europe. The direction of the train is indicated by the terminal station. For example, if you wish to go south, you will be taking the southbound train which is designated Porte d'Orleans. Interchanges are called correspondance and the two major interchange stations are Châtelet-Les Halles and Montparnasse-Bienvenüe. Free maps are available at Métro stations. The metro trains run from 5.30am to 12.30 am.
Buses
Métro could be convenient but the buses are more fun to visit Paris.
You have the option of buying a pass or a ticket. The ticket has to be punched in the
machine next to the driver. The ticket costs more if you buy it on the bus.
So, buy a carnet of 10 tickets from the metro station and use a ticket for each bus ride. You will have to press the red request button to get off at a bus stop. Buses run frequently up to 8.30pm. Some bus routes run until 12.30 am.
RER
RER is the fast suburban commuter system which has four lines running across Paris and into the suburbs. Within Paris, the RER connects some stations faster than the Metro as they have fewer stops. The four lines are : Line A (connects Disneyland Paris from Chatelet-Les-Halles) Line B, Line C and Line D. You can use the metro ticket to take the RER within Paris. The RER lines (A, B, C, D) run from 5.30am to 1am daily.