Complicated vs Complex
There is a difference between complicated and complex entities. A complicated entity or system like a machine, for example a mechanical watch, is designed in a top-down manner for a certain purpose. It has only one major emergent property, which corresponds to the desired function. Complicated entities consists of many different parts, which are combined and connected together in a special way, and all parts have to work in unison to accomplish a function. As J.M. Ottino says, they are very brittle: "One key defect brings the entire system to a halt".
Complex systems are often the result of an evolutionary and self-organizing process. Contrary to complicated systems, they are more fault-tolerant because they are adaptive, flexible and creative. A defect in an actor or element does not affect the function of the whole system. Yet the system as a whole can err, is very difficult to understand, and usually has more than one emergent property. A complex system is a system whose properties are not fully explained by an understanding of its component parts.