Complicated systems operate in standardised ways. Imprecision is diminished and non-objectivity and uncertainty are reduced as far as possible. Complicated systems can be described through non-ambiguous cause and effect chains. They are often externally controllable.
e.g. high precision machinery - engines, watches - where they are calibrated to diminish mistakes and supply certainty.
Complex systems have the presence or participation of living creatures which makes them living systems capable of change at any moment. Such systems are not easily observable nor controllable. For the most part behaviour is not predictable, and while elements may operate in standardised ways their interaction is constantly changing.