Does a 1st or 2nd Class ticket guarantee you a seat on the train?

No. A ticket, in any class, allows you to board the train, but does not guarantee you a seat. You must make a seat reservation to have a guaranteed seat on the train.

However, some trains like the TER (local regional trains) do NOT need a seat reservation. You can board the train and set anywhere you please.

NOTE: On most trains tickets and reservations are sold together as one ticket. All tickets purchased online include seat reservations where needed.

This is not the case from Train Passes. A Pass gives you a certain number of days for travel. You must reserve a seat on a train for each journey you make that day. Of course, you will only pay for the seat reservation which is about 11 €