The body uses feedback systems to control certain functions. A feedback system uses one of the products of a pathway, usually the end product, to control the activity of the pathway and to regulate the amount of that product. Feedback control may be positive or negative.
To understand negative feedback, think of how the thermostat in your house controls the temperature. Let's say that the thermostat is set at 70 degrees F (the end product concentration). When the temperature falls below 70 degrees F, the feedback system is triggered and the furnace lights and starts to pump warm air into the house. When the air in the house reaches 70 degrees F, the thermostat shuts off the furnace (no more product made; no more hot air generated). A negative feedback system maintains a 'steady state' or equilibrium and is the one most commonly found in the body.
Positive feedback systems increase the rate of formation of the product. This tends to cause change in the system rather than maintain a steady state. Think of how when a person works hard and is praised for their efforts (given positive feedback), they work harder still, expecting more praise. There are very few positive feedback systems in the body. One example, however, is lactation. The suckling action of an infant produces prolactin, which leads to milk production; more suckling leads to more prolactin, which in turn leads to more lactation. This is a positive feedback system as the product (milk) produces more suckling and more hormone. When the child is no longer breast feeding, the prolactin drops off and milk production goes down.