Paris has 3 airports (Orly, Roissy, Beauvais), what are the options to go to or to come from there?
(There is also Vatry (XCR) but there is very few flights and i see the shuttle system is currently off.)
For more details (time, maps) about the 2 first airports please check these links:
Roissybus costs 10 € and goes to Opera.
Orlybus go at Denfert-Rochereau and costs 7.20 €. (A few other stops are available, not the same depending if you are coming from or going to the airport.)
Beauvais-Tillé airport is the low-cost airport, 90 km away from Paris, used mainly by Ryanair and Wizz air.
Roissy-Charles-de-Gaulle (CDG) to Orly (ORY)
For transferring between these airports, the quickest option is the Air France coach ("Les Cars Air France"). You need line 3 ("ligne 3"). It departs twice an hour, takes around 1h30. One-way tickets costs €19 online or €20 onboard. It's advisable to buy from the bus driver than wait in line at a koisk/machine. The driver will load and unload your luggage.
Alternatively, you can schlep your bags on the RER B train. This is better if you want to stop off in the city centre. Take the RER B train from one of the two stations at CDG and change at the Antony RER station south of Paris, then transfer to the ORLYVAL shuttle or train to Orly airport. Alternatively, change at Denfert-Rochereau to the Orlybus. Its strongly recommended not to get off at Chatelet-les-Halles, one of the biggest transit hub in Europe. Very little of the journey is above-ground and is not particularly scenic. (A single ticket costs 20.30 EUR (cf. ratp.fr).