Here is a sailplane with a spanwise higher order oscillation (most likely driven by the aileron system).
I see two nodes in the right wing's motion (places where the wing is not moving) and I suspect the opposite wing is not moving in the same direction at the same time (out of phase). They were real lucky on this one because it appears to have reached equilibrium.
Here is a wind tunnel model of a well known airliner behaving in a interesting way. Not dead sure on this, but I think the yaw oscillation is driving the wing twisting as the weight of the engine pods twists the wing. Then the wing twisting causes yaw forces which further drive the oscillation.
Here is a classic flutter of a control surface. It appears to me that the port side surface is the one primarily driving the oscillation and that there is also some wagging of the rather slender tail cone. (Just learned that this is actually a RC sailplane) Obviously they knew they had a problem in the tail of this plane and the camera was put there to document it. If this had been a manned aircraft, this would be considered a high risk test and a parachute would be mandatory.
Here is an RC model aircraft experiencing catastrophic failure. As Lynn was suggesting, what happens with RC models has some relevance to real aircraft.
This model had a flying wing design and thus could pitch rapidly enough to encounter flutter even at a relatively low speed.
The last example for now is a wing in a wind tunnel with a long slender armament model bolted to a wing pylon. Here oscillations in the wing couple up with oscillations in the pylon mounted store and build with great rapidity.
The wing fails under overload in the negative g direction which is its weakest direction.
These examples are posted for use as discussion pieces.
This can be a highly technical subject, but all I'm hoping to pass on is that you will know Flutter if you see it and that you understand how serious it is.
Sid