getting from Charles de Gaulle to Versaille

There are detailed instructions on how to get the RER Line B into central Paris at http://parisbytrain.com/charles-de-gaulle-airport-cdg-to-paris-by-train/. However, don't ask for "Paris aller simple" but "un aller et retour a Versailles Rive Gauche svp". Get off the Line B train at Saint-Michelle-Notre-Dame. You then need to change to Line C. When I was in Paris last year there was a constant announcement loop in multiple languages at central area Line C stations on how to get to Versailles, but the Line C trains do not have destinations on the front but four-letter codes. To get to Versailles follow the "direction Versailles" signs to the right platform and then take a train with the code VICK on the front. There should be destination screens on the platform, but just in case you can't see them or they're out of sync look for the VICK code. The VICK train will take you direct to Versailles Rive Gauche station from where the chateau is well-signposted. However, if you're planning to make the trip as part of a layover between flights, what are you planning to do with your luggage? Also there's always the risk of a catastrophic failure on the rail network that might stop you getting back in time.

Take the RER B from CDG to Saint-Michel Notre-Dame. Transfer to the RER C and take it to Versailles-Rive Gauche. Be aware that the RER C line divides on the west side of Paris. Be sure to get on a train that is going to Versailles-Rive Gauche. The RER B line divides northeast of Paris. When you return to CDG be sure to get on a train that is going there. The ride will take 90 minutes each way. For a zoomable map of the RER & Metro system, go here.

I'm going to guess that Tim wanted to give THIS link: http://www.ratp.fr/plan-interactif/ (stupid, lazy links; I think they're cats...)

Sounds like a dash on a layover. If I'm right, take care and allow for delays. Don't miss your flight because you think you can do that extra thing if you only have 15 minutes more....

or you could simply take an Air France shuttle to the Montparnasse station and hit the train titled "Paris - Chartres - Nogent le Rotrou - Le Mans". make sure it's listed on that train's stop.