Public Transports
RATP: (English and French).
Paris is well equipped in public transports. Administered by the state owned company RATP. You can get a map in any metro station. With one single metro ticket you can go anywhere within Paris city limits as long as you don't go out of the station where you are connecting from one line to another. That goes for Metro and RER, the ticket policy on the bus working according to sections. According to the length of your stay you better buy a pass called "Carte Orange" valid for a week or a month which gives you access to different concentrical sections from Paris city-limits towards the far suburb with intermediate sections according to your needs.
Paris Visite Pass
Travel in Paris and its suburbs for a 1, 2, 3 or 5 day period.It can be bought at any mainline metro or RER station (Half-price Paris Visite cards are available for children under the age of 12).

The Metro
Paris has an excellent and very extensive public transportation system. You're able to cover the majority of the city in a matter of minutes.
It covers the entire city and it's not expensive. It's almost always underground, which doesn't allow you to take in much of the city sights, and it's generally crowed during the rush hours (8-9 AM and 6-7 PM).

The tickets are sold in all the metro stations and and in almost all the bureaux de tabac. A ticket costs around 1,5€, no matter what distance you travel in Paris and the nearby suburbs. You can also buy your tickets in a packet of 10 called a "carnet" (pronounced kar-nay) and which costs 12,00€. If you're travelling to the outer suburbs, the price will depend on your final destination.
If you plan on using the métro to get around Paris, buy a pass. They have them for several different time durations (1 day, 2 days, 3 days, 1 week, 1 month, 1 year) and they're a lot less expensive than buying your tickets seperately. Starting with a week pass, you'll need an ID photo in order to buy your pass.

The RER
The RER (Réseau Express Régional) is designed primarily to connect the city of Paris with its suburbs. The RER is accessed in using a metro ticket or metro pass, as long as you're travelling in the Paris city limits (or its nearby suburbs). Beyond that, the ticket prices are proportional to the distance you're travelling.

Outside of the Paris city limits, a lot of the RER trains do not stop at all the stations You need to check before getting on a train to see if it will be stopping at the station you're wanting to go to. Each train is identified by one of four letters: A, B, C or D, E plus an accompaning number, and which determines its destination and the stations where it'll be stopping.

Bus
Access to the buses is done in using the same tickets or pass that you use in the métro. A ticket is only good on one bus. If your itinerary uses several bus lines, you'll need a ticket for each one of them. If you're using a pass instead of individual tickets, all you need to do is show the pass to the driver when boarding the bus.
There are a few general rules to know about the bus number.
- The buses numbered 2X go to Gare Saint-Lazare
- The buses numbered 3X go to Gare de l'Est
- The buses numbered 4X go to Gare du Nord
- The buses numbered 7X go torwards the center Paris
- The buses numbered 9X go to Gare Montparnasse
A bus marked with with three numbers is a connection with the suburb.

Batobus.
Discover Paris by the river Seine. The boat stops at 7 stops along the river, when you embark, and disembark as often as you like.

Night Transportation.
The public transportation in Paris stops during the night. Many of the buses stop around 8PM and restart again at 6AM, and others stop around midnight. As for the metro, the hours vary, but in general the last métro departs from the beginning of the line around Midnight. The first train leaves around 5.30AM.
There exists a night bus, called the Noctambus, which runs all night with a departure every hour from 1.30 to 5.30 AM. These buses connect the center of Paris, leaving from Chatelet, with the various entrances to the city. The night buses are more expensive than the normal day buses.