Feedback loops
A feedback loop is a circuit that feeds back some of the output to the input of a system.
Negative feedback loops keep things in check, or out of control. For example, body temperature. If your body is too cold, your body automatically starts shivering to warm up. Once you warm up, you stop shivering. Same with if you get too hot. You start to sweat, which causes you to cool down.
Positive feedback loops allow things to get out of control. For example, A nuclear chain reaction. Once a reaction starts, it is self-sustaining in that the products of the reaction also causes the reaction to continue- until there is no more reactive.