Phrases like grams per meter squared per month, or pounds per acre per year refer to these transfers. Driving variables are elements that influence a system directly, but are not classified as residing within the system. Scientists use this designation to limit discussion and avoid confusion. Say, for instance, you are conducting a study on the grassland behind your house. Obviously, sunlight directly effects the system as you have defined it, but if you chose to include the sun's light as part of the system, you very well might spend years studying solar flares and never get around to more practical things like what yearly irrigation does to the land.
Feedback is a term used to describe the interactions and reactions of components within a system.
A simple example of a feedback is the control of the speed of a pick up truck exerted by the driver. The truck and it's driver can be considered a system. The speed is controlled by the driver depressing or releasing the accelerator. For example, if we are trying to grow a garden in semiarid region of the southwest, we know we have to water it. Other controls of garden success are fertilizer, insect pests, weed pests, pruning and fruit removal, type of seeds planted, and many other factors.