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. 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.