From Unofficial BOINC Wiki
Sometimes an output can also be one of the inputs to a model or system. In this case it is said to be a Feedback. A common example is when a microphone picks up some noise from a speaker. The effect can be a runaway effect where the noise is repeated so it gets louder so the microphone picks up more noise and all you hear eventually is a loud screech. Note that it does not necessarily have to involve a runaway effect - if the feedback works in the opposite direction negative Feedback it can help the system to be stable. Also even if a feedback is positive it need not involve a runaway effect, it just might not be strong enough to overcome other stabilising effects.
A typical example with climate models is water vapour. Water vapour acts as a greenhouse gas. So increased temperature can increase water vapour through evaporation and the greenhouse gas effect could cause increased temperature, which in turn could .........
So is Water vapour a Forcing or a Feedback or both? It is called a Feedback and not a Forcing because water on average only spends about 11 days in the atmosphere before being rained out. This is too short a period for it to have any noticable forcing effect.
 Also See
Things that affect the outcome of climate models include:
- Parameters Settings used by the model.
- Initial conditions - Initial condition do affect the weather but generally initial conditions are found not to affect climate much. Initial conditions can be considered to be a Parameter.
- Forcings Changes like greenhouse gas levels that force the climate to change.
- Feedbacks Things like water vapour (which is a greenhouse gas) but the level is determined by the model rather than forcing the model.