Positive feedback loops enhance or amplify changes; this tends to move a system away from its equilibrium state and make it more unstable.
Negative feedbacks tend to dampen or buffer changes; this tends to hold a system to some equilibrium state making it more stable.
Several examples are given here to help clarify feed back loops and to introduce loop diagrams.
Understanding negative and positive connections is helpful for understanding loop structure.
An example of positive feedback is world population with a fixed percentage birth rate. Positive feedbacks will result in unlimited growth (until checked) and are sometimes referred to as vicious cycles. In the figure below connecting population to births, large populations cause large numbers of births and large numbers of births result in larger population. This idea can be modeled nicely with the differential equation dP/dt=+rP, where P is population and r is the percent birth rate. The solution to this is P(t)=Po(exp[rt]) or exponential growth. Not all positive feedbacks give exponential growth but all, left unchecked, will result in unlimited (or unstable) growth. In the graph below we show the world population predicted for a fixed 2% growth rate from 1950 to 2050. Also shown is an estimate of future world population which is close to the mid-range United Nation Environmental Program (UNEP) best guess for future population to stress that exponential growth is not realistic for world population although it works fairly well for the time between 1950 and 1990. Logistic (S-shaped) growth would be a better choice for modeling world population for this 100 year time interval shown. UNEP Population