In biological systems, such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimum level under certain environmental conditions. The deviation from the optimal value of the controlled parameter can result from changes within the environment or without the environment, internal or external.
Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, positive and negative feedback do not imply consequences that are good or bad in effect. A negative feedback loop would be one that slows down a process, while a positive feedback loop would be one that accelerates the process.
An example of negative feedback would be the accumulation of lactic acid in a weight-lifter's muscles, causing the muscles to slow down in their activity. An example of positive feedback would be the hormones secreted to make a person blush. Upon discovering they are blushing, the process is accelerated, causing even more blushing, due to embarassment.