Negative and positive feedback are the factors in a process that drive the system either toward or away from a stable configuration, according to Dictionary.com. One example of a homeostatic process involving negative and positive feedback is the Earth's climate. Homeostasis is the tendency of the system to push back toward a stable state in response to external influences.
Positive and negative feedback are central to an understanding of homeostasis, which About.com describes as one of the unifying principles in the study of biology. Examples include the regulation of the body's temperature, blood pressure and the body's water and electrolyte balance.
Howstuffworks.com describes a mechanical negative-feedback mechanism that was built into early steam engines. The fly-ball governor was a freely rotating set of swing arms with weights attached to their tips and mounted on a threaded post. As steam pressure built up inside the boiler, steam would be forced up the post and cause the arms to swing. As they swung faster, the weighted tips would rise and force open a valve to release steam. This lowered the pressure and brought down the governor's arms, closing the valve again. This is a classic example of negative feedback maintaining a constant pressure. Reversing the effects by closing the valve when the arms rise would be an example of positive feedback in which high pressure induces higher pressure still until the system breaks down.