I am purchasing tickets for Paris to Avignon,and have never travelled on the SNCF trains in France. What is the difference between 1st and 2nd class, and is it worth the difference? Also, is there a dining car on the train?

Personally, I don't think there's enough of a difference between first and second class on the TGV to warrant paying the higher price. The seats are roomier, and on some TGVs you have your own small compartment, but the regular seats, whether they are seats or compartments, are just fine. And it's only a 3-hour trip.
There's a car on every TGV that serves food, but it would be a stretch to call it a dining car. You can get coffee and water and soft drinks and sandwiches and salads and candy bars. I usually buy something at the train station to eat on the train - better selection there, better quality, and lower prices.

The main difference between first- and second-class seems to be that second is usually more crowded. The "cafés" sometimes offer hot meals as well as sandwiches, but as St.-Cirq says you can do very well by picking up food at one of the station counters.

save your moneys and travel 2nd class.

Beyond the seat size / leg room, first class tends to be quiter since there are less people and there tend not to be screaming children or loud student groups etc. Sometimes in first there are powerplugs at the seats for laptops etc but thats a real crap shoot depending on when the actual train car was built or refurbished.

That said if you are planning ahead and can get the PREM fares that StCirq has mentioned here many times its really hard to justify the big difference in price. However, if you are booking at the last minute and can get only full price in 2nd there are often discounts still available in 1st which makes it about the same price or only a little more.

If the extra cost of going first class won't prevent you from dining/staying at better places, then by all means book it, it is more comfortable as mentioned by previous posters. Now, courtesy of Private Eye, consider the difference between the classes on trains in India:

"It's not an issue of second-class passengers making do with less light, but a matter of affordability," Mr Pandley of Western Railway told reporters in Mumbai. "This is pure economics, and it is not at all unfair. From now on, our first class compartments will be fitted with hundred watt lights, but the second class compartments will only have sixty watt lights. Considering that an average second class fare is one-third of a first class fare, they ought by rights to be getting only thirty-three watts. So in reality, out administration is being kinda and generous in giving them almost double that power, with no matching increase in fares."

Asked if this policy would be extended to other amenities, Mr Pandey continued, "Sure. We have a number of ideas which will be taken up in stages for implementation. take the case of fans. The second class already have fewer fans, naturally, but a commuter standing under the fan in second class currently receives as much air as a first class passenger similarly placed. So to reflect the difference in fares, we are planning to reduce the speed of fan rotation in second class compartments, and our engineering department i currently modifying their design."

Thanks everybody. I was able to get the "Prem's" rate of 25 euros each, for 2nd class. I really appreciate all of your help!!