Flutter is exactly that, a "flutter" or oscilation of the aileron during flight that is usually noticed due to the "rippling' sound it makes. It can also happen to the elevator and rudder. There is some debate as to its true cause, and I believe there can be more than one cause, but it can definitely bring your airplane down. The usual suspect is too large a gap in the hinge area between the aileron and the wing, but the construction of the aileron can also be part of the problem. I've only had aileron flutter occur on one plane so far. It was a Stick .40, and it only occurred comming out of high speed dives. I believe it was due to ailerons on this plane having a very large covering surface stretched over a minimal frame structure. This allowed too much flex in the covering and allowed a "billowing" effect in the covering that caused the "flutter".