What are positive and negative feedback loops?

Feedback loops are a series of events which are triggered by an initial change. 
Positive feedback loops are feedback loops which amplify the initial change and lead to a repetition of the cycle. An example of a positive feedback loop could be an initial cooling of the earth. This leads to the growth of ice sheets, which are reflective, meaning more solar radiation is reflected away from the earth leading to further cooling of the earth. Thus the initial change is amplified leading to more ice and so on. 
Negative feedback loops are feedback loops which counter the initial change normally leading to the conditions found before the initial change. An example of a negative feedback loop is how the human body regulates its own temperature (homeostasis). If the body is initially warmed then it will sweat and blood vessels will expand leading to cooling thus countering the warming and restoring the temperature of the body to what it was before the temperature increase.