PARIS

So far if u ever think to go backpacking to France i think it should be no problem in term of transportation, all u need to know is the nearest station to your destination.

There are lots of metro and RER line in Paris, click the above image and you will be redirected to the Metro and RER map website.
to save your money you can plan your journey up front by studying target destination as in Paris you can buy the ticket in piece or bulk. one piece of METRO or RER ticket for journey inside Paris zone 1 will cost you 1.60 euro but if you buy in bulk of 10 pieces (it is called carnet) it will only cost you 11.60 euro. the ticket(s) can be be bought from the vending machine located at the entrance of the METRO or RER station. but be prepared with coins as some machine do not accept notes for ticket transaction. most of the tourist attraction is located inside zone 1 and the ticket is valid for 90 minutes from the first time usage. there are certain rules for the ticket

• valid for 90 minutes from first time usage
• can be use for transfer from Metro to Metro, RER to RER, Metro to RER, RER to Metro, Tramway
  
• can be use for Monmarte Funicular
  
• but it is not possible for transfer from Metro or RER to BUS or vice versa (just train to train or bus to bus)

For your info, there is RER linking the CDG airport to Paris and it will cost you about EUR8.50. If you are carrying lots of bags i recommend you to take the bus, if not the RER is a lot more cheaper.