Paris (in its walls, that is inside the roughly oval area encompassed by the Périphérique) is only 12 kilometres wide and 10 kilometres across. 

The transportation system of Paris is mainly run by two state-owned corporations : the RATP and the SNCF-French railways. RATP oversees métro, bus and tram networks, as well as some part of the RER-express train network. The SNCF runs the rest of the RER network and the Transilien suburban trains.

The fare system is wholly integrated : any ticketing booth of either RATP or SNCF will sell you the whole range of tickets and passes.

The best way to get around in Paris is to walk and use the excellent public transportation. Driving would really be a bad idea, as the traffic is usually very heavy in the city and parking is a nightmare.  As stated before, the public transportation is amazing.  It enables easy and fast connection to anywhere in Paris and surrounding areas.  The Paris metro is very wide spread and comes every 2 minutes during rush hours. It may be a bit crowded, but at least you will always be sure to get where you want quickly. If you want to experience a high-tech automatic metro line, try the number 14. It is the latest metro line, incredibly clean and, strangely enough, well-scented! However on the RER connection stations in Paris, beware of the high speed people mover.  It is a flat people mover that moves at an incredible speed.  You must be prepared before stepping onto it or you will either fall or hurt yourself.  

The Métro ceases to run at 1.00am. If you find yourself stuck in a station after hours with the gates shut; and want to get out, sometimes there is a gate switch on the wall of at the interior side of the gate.

Since 2005, there has been a better service after midnight. Two circular bus lines run between Gare de l'Est, République, Bastille, Gare de Lyon, Austerlitz, Montparnasse, Invalides, Champs Elysées, Gare Saint Lazare, Pigalle and Gare de l'Est every 17 minutes at weeknights and 12 minutes at weekend nights. A number of radial night lines go through the city and some go into the farthest suburbs (such as CDG airport) . Passes and regular tickets are valid. On the whole, the service is efficient ; although tourists and guests might find the ambience of some night buses a bit unsafe. To be sure to go back to your accomodation as fast as possible, the best thing is to check which night bus line you have to take. It is possible to do it directly on the RATP website. 

Since 2007, you can also get around Paris by bikes with the Velibs . Those bikes are in free access : you can buy a ticket for one day (1.70 euros) or a week. It is really cheap, and easy, and it is such a good way to see Paris without getting to tired. There are so many stations in Paris (every 300 meters) so you will not have any problems to find one!  You just have to switch of bike every 30 minutes otherwise you will have to pay one more euro. Just pay attention with the traffic, because some parisian really do not drive carefully !