A single metro ticket costs 1.5 euros so if you are staying for time in Paris it becomes worthwhile to buy a Carnet, which is a book of 10 tickets for 11.10 euros. A single ticket from a Carnet, can be used in either METRO / RER / Bus /Tram lignes.
The airports (CDG, etc) are all outside the carnet zones, so a higher price ticket must be bought.
Carnet can be purchased from any Metro station, ask for une carnet s’il vous plait. They can also be bought from vending machines with cash or credit/debit cards that have the embedded chip. Smaller stations have no ticket office, only vending machines. Tabacs with the RATP logo displayed also sell carnet.
A carnet is not a travel pass, it is a book of ten tickets, it has no expiry date and tickets will remain valid for many years, the tickets can also be used by fellow travelers, so two or three people can share the Carnet (10 tickets, not a single ticket out of a carnet !).
A single ticket will take you anywhere within the two central zones of Paris, and you can now transfer between the Metro, and the RER (or Bus and Tram) on the same ticket. But please note that tickets bought from bus drivers do not permit transfer to Metro or RER (or Tickets used in Tram is not valid in Metro or RER), so purchase ahead if you wish to transfer. Once the ticket has been activated you have 90 minutes to get to your destination.
**CAN** transfer between
METRO lignes and RER lignes
Bus and Tram lignes
between Bus/Tram and Metro/RER lignes.
One exception to the 2 central zones in Paris restriction, is the number 1 line to La Defense. Even though La Defense is in zone 3, a regular metro ticket will get you there.
A Carnet can be bought on the Eurostar, to speed things up when you get to Paris.