Many molecular and physiological processes are controlled by feedback mechanisms.
In a feedback loop the product of a process, such as the breakdown of proteins into amino acids, has an effect on the rate of the process.
Negative feedback occurs when the rate of the process decreases as the concentration of the product increases.
Positive feedback occurs when the rate of a process increases as the concentration of the product increases. Negative feedback controls the rate of a process to avoid accumulation of a product. The rate of a process will continuously accelerate under positive feedback as long as substrate is available and the product is not consumed by some other process.