What is the difference between Negative feedback and Positive feedback mechanisms?

In negative feedback, the response to the stimulus tends to decrease or eliminate the stimulus.
An example is the thermostat in your house.  There is a monitor, which has a set point (the temperature to which you set your thermostat).  If the temperature goes below the set point, the monitor triggers the effector (your furnace), which turns on the heat.  The stimulus was the cold; the heat turning on gets rid of the cold.  

Negative feedback systems are generally used to maintain homeostasis in an organism.  

In positive feedback, the response to the stimulus tends to increase the stimulus.  The body uses positive feedback systems when it wants a large, fast response.