Open systems (ecological, biological, social) contain many types of regulatory circuits, among which positive and negative feedbacks. Positive and negative does not mean desirable or not. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to accelerate it.
When a change of variable occurs in a system, it reacts to regain its equilibrium.
In the negative feedback, the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it is stabilizing and allows to maintain homeostasis.
See also:
wikipedia.org dumped 2003-03-17 with terodump