From
http://www.centennialofflight.gov/es...ty_II/TH27.htm
Dutch roll is a motion exhibiting characteristics of both directional divergence and spiral divergence. The lateral stability is strong, whereas the directional stability is weak. If a sideslip disturbance occurs, as the airplane yaws in one direction, the airplane rolls away in a countermotion. The airplane wags its tail from side to side. You can also read:
http://en.wikipedia.org/wiki/Dutch_roll It is short and sweet.
For the true aerodynamic explanation, go here:
http://www.av8n.com/how/htm/equilib.html