TER recieve some cascaded stock from previous SNCF operations. For example, the Corail coaches -ordered in large amounts in the 70s, have been sold onto various TER operations from medium distance services. http://en.wikipedia.org/wiki/Corail_(train)
Over the past 10 years or so, many of the 70s units have been replaced by new-builds from Alstom or Bombardier.
Hope this provides a little bit of an insight. On family holidays we've done a bit of rail travel in France (TGV, Corail and TER D/EMUs) so any further questions feel free to ask