The earth's climate system is directed by feedback loops (also known as feedback cycles). A feedback loop is a circuit-like system in which an initial disturbance acts as an input into the system. This creates a change in some other component of the system, known as an output, which then influences subsequent occurrences of the initial disturbance.
In a positive feedback loop, the output of the system serves to reinforce or amplify the initial disturbance. In this way, A produces B which creates more of A.
In a negative feedback loop, the output of the system acts to restrict the effects of the initial disturbance. In this case, A produces B which reduces the amount of A.
Keep in mind that a "positive" feedback loop is not necessarily desirable, just as a "negative" feedback loop in not necessarily undesirable. These words refer to the direction of change, not the desirability of their effects.