Metal parts inside, depending upon their surface area, thickness, composition, and proximity to the source of the heat, all warm up at different rates, making pre-heating necessary, so that all of them get to approximately the same temperature. This creates various heat zones within the oven. All ovens overcome the heat zone disparity, to some extent, by using dampers to control the flow and accumulation of heated air, in order to provide as uniform a temperature as possible. Convection and rack ovens force air through the oven, using carefully controlled dampers, deflectors, and air-flow rates to provide more temperature uniformity. Convection ovens are to baking what wind-chill is to being outside on a cool day.
Convection baking is the same principle in reverse.
Introduce fast moving air into the oven and the insulating air is blown away, allowing the heat to penetrate much more quickly. Many recipes and handlings are based on ovens which do not employ forced air movement. As a rule of thumb, bake to time and color and adjust the temperature so that you get the right color in the prescribed amount of baking time. The greater the volume and rate of air flow in a forced air oven, the lower you need to go with your temperature setting. Rack ovens generally move air more for the purpose of temperature uniformity than for quick baking.