Both buffet and flutter is a vibration of a surface of the aircraft, such as a wing or tail, due to the air flowing over it (especially if the air is turbulent). The difference, I would say, is;

 - If the surface does not exhibit an increasing amplitude, as it is naturally damped at the frequency of the excitation force, it is called **buffet**.
 - Conversely, if the surface is not naturally damped at the frequency of the excitation force, its amplitude of oscillation will continue to grow, and likely structurally fail, catastrophically, it is called **flutter**.

As examples of each, here is a video of [flutter.][2] As can be seen, the amplitude of vibration starts to grow, and it wouldn't appear surprising if the tail plane failed very quickly.

In comparision, the F-18 has experienced [tail buffet][1] from vortices from the wing leading edge extensions. As can be seen, the tails vibrate, but its a much smaller oscillation than the flutter example. While it could cause cracking, it would be expected to fail instantly. 

  [1]: https://m.youtube.com/watch?v=9QOCjrP8dE0
  [2]: https://m.youtube.com/watch?v=iTFZNrTYp3k