There are 2 major international airports around Paris: Charles de Gaulle (a.k.a. Roissy) located north-east of the city, and Orly, located south of the city. You can get to either airport either by taxi or Air France coach for € 15 to € 70, depending on where you start from and where you’re going.
Both airports are run by the Paris airport authority. An excellent image outlining all your options is available here (french only). What follows gives you a general overview of how to get from/to the airports by public transit.
Charles de Gaulle
… by bus
There is a special shuttle (city bus) called RoissyBus which will take you from the CDG airport to the Opéra métro stop for € 9.40. There are also two regular bus routes during the day (#350 from Gare de l’est, and #351 from the Nation métro station), and two night routes (#N140, and N143). Both buses will cost you three (3) zone-1 tickets.
… by train
The quickest and most relaxing way (in my opinion) to get from/to the airport: the RER B line (€ 8,70). Keep in mind this is a commuter-train, so if you are travelling during the work-week the train will be quite full with commuters and there is not much room for luggage.
A quick note on taking the train from CDG:
* There are many automatic ticket dispensers at the CDG train terminal, however North-American credit cards are notoriously difficult to use with these dispensers if they do not have the chip technology (if you have American Express, forget about it)
* If you have to buy your ticket from an employee, the lines are impossibly long. If you’re lucky enough to know someone who lives in Paris, the best thing to do is have them send you a Paris-CDG ticket in the mail before your trip. It will save you a lot of time.
Orly
… by bus
Similar to CDG, there is a bus service (OrlyBus) to take you from the Denfert Rochereau station to the Orly airport (€ 6.60). You can also opt for regular city-bus service from/to the airport with bus routes 183, 285, and 292. Night bus: N31, and N131
… by train
Both the RER ‘B’ and ‘C’ lines pass close to the Orly airport, but you will need to take a shuttle to complete your journey. Use the OrlyVal driverless light rail trainsit line to get you from the airport to the Antony train stop (RER ‘B’) for €10.25, or the “Paris by train shuttle” to get you from the airport to the Pont de Rungix Aéroport d’Orly train station (RER ‘C’).
Summary:
In my opinion, getting to Paris from an airport by train is the best way to go, but there is one disadvantage: all the stairs in the Paris métro! Once you get to Paris by train, you might want to complete your journey by taxi if you have a lot of luggage with you.

