*Sorry I don't have time to go in depth on this, but to get things rolling I'll give a basic description.*
Initially Feedback Loops enhance or buffer changes that occur in a system.
A positive connection is when a change (increase or decrease) in a variable causes the same change in the second variable (increase/increase or decrease/decrease). Since it is a direct proportionality it is considered positive.
A negative connection is similiarly when a change (increase or decrease) results in an inverse propotionality change in the second variable (increase/decrease or decrease/increase).
When these two connections are combined you have a negative feedback loop due to +(-) = - One must multiply all the signs of all the connections around a loop to find out wether it is a positive or a negative feedback loop in more complicated loop structures.
In general Negative feedback loops tend to dampen or buffer changes, tending to hold a system in an equilibrium state, making it more stable.
Positive loops on the other hand will result in unlimited growth (until checked) and are considered vicious cycles. So in general positive feedback loops enhance or amplify changes, tending to move a system away from an equilibrium state, making it more unstable.
Hope that helps get things started