They are regional trains (TER). Normally there's no price advantage in buying ahead with TER, but I see that if I look at a dummy March booking and a dummy tomorrow booking, by booking ahead I could get a 10€ ticket instead of paying €18.40 — but booking ahead locks me into a specific time and date and is not reimbursable. For €8.40 a ticket, I would just buy one when I got to the station.
There are trains from Montparnasse just about every hour; it takes one hour or an hour 15 minutes for the journey. (I have a colleague who lives in Chartres and commutes to Paris, so do be mindful of rush hour.)