Based on what I have read on this site, when you buy a ticket from TGV, you have to pick up your ticket at one the train stations in France. No option to print them online. Yet, if you book a ticket from DBBahn (for a trip from France to Germany, for example), you can print your ticket online. I have to assume that this online ticket is accepted on the train. Does anyone know why this is so?
As Steve said, each national rail company has its own way of doing things.
If you book at tgv-europe.com and get a Prem's fare on a TGV and certain Corail trains for travel entirely within France, then you often can print your own ticket. Otherwise you indeed must pick up the ticket at a SNCF station or boutique.