The fan inside a convection oven increases its efficiency by creating air currents to distribute heat evenly throughout the cooking space. This reduces the amount of time and energy needed to preheat the oven, and also reduces the cooking time of food due to the improved transfer of heat.
An ordinary oven relies primarily on radiation to transfer heat into food, although uneven heating inside an oven creates small convection currents. The area around the heating element warms first, and then that heat slowly radiates throughout the oven until the oven is more or less the same temperature throughout. The fan in a convection oven speeds this process up by circulating air across the heating element, spreading warmth quickly and evenly throughout. In addition to speeding up preheating and cooking, this also tends to reduce hot and cold spots that can cause foods to cook unevenly.
Cooking with a convection oven takes some adjustment for those used to traditional ovens. Since the heat transfer is more efficient, cooking times or cooking temperatures should be reduced slightly to compensate. In addition, while many baked goods cook more evenly in a convection oven, the fan-forced air reduces the humidity inside the oven and alters the cooking environment, which can reduce the quality of breads and cookies produced in a convection oven.