Metro/RER

Best way within the city (NOT including the CDG or Orly airports) is to get a carnet of 10 metro tickets and use to come and go as you please.  Obviously if you don't use some of the tickets they're not refundable.  So if you won't use 10 tickets, just get individual tickets.

AIRPORT to CITY: RER (regional express train) - it's cheap, quick, and very comfortable.

NEVER a bus - it takes ages if it even arrives.  Sometimes it will arrive but won't stop, so you'll just have wasted loads of time (bec buses are perpetually late).

There's ticket machines in the both terminals of CDG (section D in terminal 2). They operate only with coins (euros) or credit cards.