You can take the RER Line B suburban train from Paris-Charles de Gaulle (CDG) Airport to central Paris in as little as 35 minutes for 8.70€. (You can also use it to return to the airport, but read this important note.)

CDG Airport has two train stations, Aérogare 1 and Aérogare 2. (Note that these are not the same as Terminals 1 and 2!)

Aérogare 2
Aérogare 2 is the main train station at CDG Airport, serving both RER Line B suburban trains to Paris, and SNCF TGV intercity trains to all parts of France. It's located between Terminals 2C and 2E in the gigantic Terminal 2. If you follow signs in the airport for Paris par train (Paris by train), they will lead you to Aérogare 2. Signs pointing to Gare or Gare SNCF also point you toward Aérogare 2.

Aérogare 1 - Roissypôle
Aérogare 1, south (ie, toward Paris) from Aérogare 2, is next to the Roissypôle office and airport hotel area, and the Roissy Gare Routière (bus terminal). Aérogare 1 is a 5-minute walk from CDG Terminal 3.

These train stations, and CDG Airport's Terminals 1, 2 and 3, are connected by the CDGVal Airport Shuttle, a small, free automatic tram that also serves several airport parking lots and the Pullman Hotel. More...

The Paris par train signs throughout the airport may lead you to the CDGVal, which then takes you to Aérogare 2.

Here's how to buy your ticket and choose your train at Aérogare 2.

(You can also buy your ticket and enter the RER system at Aérogare 1, but most travelers will use Aérogare 2.)