This diagram is from a white paper written on the current financial crisis. The R's mean reinforcing feedback, the B's are balancing feedback, the S's (same) are where more of one thing lead to more of another, and the O's (opposite) are where more of one thing lead to less of another.
The most interesting effects occur when you have a delay in a loop, where you initially don't know if the link does anything or not, you have to wait.
