Reservations on trains

Yes, sometimes.
Each country, and each railway company have a different set of rules and a different way of doing business, but the are some generalisations.

1) Regional trains, also known as local trains. No reservations, you buy your ticket (from the manned ticket counter or from a ticket machine); and it is valid for any train. Just get on the next train, find a free seat and sit down. In Belgium, Netherlands and Switzerland nearly all trains work on this principle. There is mostly no financial incentive for buying a ticket early, it is a fixed price.

2) High Speed trains. This includes all French TGV trains. In this case a ticket is for a specific train. When you buy a ticket you will be asked which train on which day you wish to travel on. You will then be assigned seats on that train when you buy the ticket. It will say on the ticket which train, carriage/coach and which seat you have. Your ticket is only valid on that train.
You can by a ticket on the day, or buy it weeks or months in advance. But, big but, the earlier you buy the ticket the cheaper. If you buy the ticket on the day you pay "full price", if you book earlier it is cheaper, sometimes 1/3 or 1/4 the cost. But, if you change your mind, you have to buy a new ticket, and the old one is waste paper.
This is the same pricing model as airlines use.

Whether or not a train requires a seat assignment depends upon the train, not usually on whether you have a point-to-point ticket versus a rail pass. TGV encourages advance booking, but there is no deadline to buy a ticket if you're willing to pay full fare. To travel by TGV in France with a rail pass, the number of places for pass holders are very limited, so you do have to reserve further ahead.

Some unreserved train tickets, such as Paris-Bayeux, would have added date/time restrictions imposed if you chose an advance discount fare online, even though a seat assignment is not required/included in full fare tickets. You can buy tickets at the station when, instead. See also http://www.ricksteves.com/travel-tips/transportation/trains/buying-tickets.

The DB train schedule site is a good, impartial indicator of which train legs are "subject to compulsory reservation." You see more detail when you click the arrow to the left of any train schedule.

TGV trains can and do sell out on popular routes so it is quite possible that even though you may be willing to pay the full fare that you will not be able to purchase two seats.

Thank you everyone. Reading your responses has discouraged me from going to France. Traveled by train in Italy and always bought last minute tickets with no trouble and no pre reservations. Here is my tentative itinerary to see if it leaves me no other choice but to deal with this problem or pick another country.
Day 1: Arrive in Paris morning, train to Strasbourg to spend day, train to Colmar for nite stay.
Day 2: Colmar
Day 3: Colmar, late afternoon train to Beaune
Day 4: Beaune, late afternoon train to Annecy
Day 5: Annecy
Day 6: Annecy
Day 7: Morning train to Chamonix
Day 8: Chamonix, late train back to Annecy for over nite stay.
Day 9: Morning train to Paris
Day 10: Paris
Day 11: Paris
Day 12: Flight back to Chicago

Doable? We are a couple of 65 yrs and looking for relaxation, scenery, quaint villages, good food, & wine. In Paris just interested in Eiffel Tower, Sacre Couer, Louve, Moulin Rouge, cruise on River Seine, and people watching. Thanks all!

Please don't give up on France because of train travel ticketing . We are in our 60's as well and traveled to France for the first time 2 years ago. We found the booking and traveling on the trains very easy and relaxing. We took about 5 train trips from Amsterdam to Paris to Avigion and to Beaune. I did my booking ahead at Loco2 website before we left and printed most my tickets before we left. We are planning another month traveling in France next year and will be using the trains again. Booking ahead will give you some very good price breaks.

High speed trains in both France and Italy have reserved seating and could technically sell out. Just like in France, train travel in Italy on high speed trains becomes more expensive as the date of travel approaches and less expensive tickets sell out. Both countries have slower regional trains that do not require reservations, do not sell out (meaning you stand if you can't find a seat) and have no fluctuation in the ticket prices. So, if you were able to get comfortable with the Italian train system, you should be fine with France. If you are willing to pay the full fare, then just wait like you did in Italy and buy your tickets on the date of travel. Paris to Strasbourg can be as high as 96 euros per person for a second class seat if bought on the day of travel to as little as 25 euros for the exact same seat if purchased 90 days ahead, but that ticket is probably going to be non-refundable. Ultimately, it is the traveler's decision if he or she wants to save money and be tied to a particular schedule or if the preference is to travel more spontaneously and purchase tickets on the day of travel.

While sometimes a train will sell out, there are multiple trains per day. Except perhaps at a major holiday weekend, you will be able to buy a train ticket a day or so in advance.

Thank you Sondra, JHK, and Douglas. I appreciate all your advice & info. I've just started to put this trip together and still have a lot of research to do. I will keep in mind everything you all have told me and maybe, as time gets closer and my knowledge grows, I will not be so concerned about France's train system. Thanks again!

Linda, I took trains all over France a couple of years ago, and like you I was familiar with the Italian train system. It's easy, the main difference for me was that, when buying tickets online for France, some of my options were limited in terms of where to pick up tickets (a few did not have an e-ticket or print-at-home option). I always checked "pickup at French station" or however that is worded, the picked up all my non-e-tickets at my first train stop.

One warning is that it in France the slower regional trains sometimes don't provide an alternative to TGV services. On some routes there are slower trains all the way, but on others they are deliberately broken up and timed to provide poor connections, to force people to use TGVs.

Thank you Zoe & Philip for the valuable information. Starting to settle into the thought of just deciding on the train to take ahead of time instead of winging it. I see now how much money can be saved that way. Trip still a ways off (not planning until late May or early June 2016). So by beginning of next year, I'll research to see the trains available for our itinerary and cost of each one. That should help me decide if France will be the country for us or somewhere else. My heart always goes back to Italy but have always wanted to vacation in France.

Your trains to and from Paris are the longest, most expensive, and most likely to be served by TGV. If you are thinking of taking a train from CDG airport to Strasbourg, the most convenient is a direct TGV departing just a few times per day, such as 10:20, 12:50 and 18:30. Since purchasing ahead would lock in a date and time, you'd have to allow some cushion for flight delays.

Instead, look at flying into Strasbourg airport (or Mulhouse/Basel/Freiburg airport) and out from Paris using a "multi-city" plane ticket.

You can "fly" straight to Strasbourg with Air France. The "flight" from Paris to Strasbourg is via TGV but you buy one ticket from Air France that includes the train. The good thing about that is that if your flight is delayed and you miss your train, Air France is responsible for getting you on the next available train. Take a look at http://www.airfrance.us/US/en/common/resainfovol/avion_train/reservation_avion_train_strasbourg_airfrance.htm.