"Rather than let hot air circulate randomly, convection ovens carefully create a uniform temperature with internal fans that circulate hot air. Convection ovens are larger and more expensive than standard, or radiant, ovens, but they cook food faster, at a lower temperature, and with better results. Fans ensure that the same temperature reaches the top and bottom of foods, as well as foods at all rack levels.
A frequent complaint of cooks with radiant ovens is that bottoms of foods get scorched, while tops are not browned evenly. This is because the temperature is not the same over the course of the cooking time, as well as over the volume of the oven cavity. Convection ovens correct this variation by using a fan that blows the hot air throughout the oven. A "true" or "European" convection oven goes one step further by adding a third heating element. Thus, the fan actually blows pre-heated air, rather than distributing the already-heated air. These are the most expensive and effective types of ovens.
Cooking with a convection oven requires some adjustments, but proves much easier and more rewarding in the long run. Because the heated air transfers heat more efficiently to cooking containers and exposed food surfaces, food will take less time to cook. Most recipes can be cooked for 25% shorter time, which ends up saving energy. Also, you might need to slightly lower the temperature at which food cooks on a trial and error basis.
You'll notice that convection ovens seal in the juices of meat so dishes taste more flavorful and moist. Baked goods, such as pies or cookies, will be perfectly browned, even if you place them on different racks. Pastry will come out better, too, because the heat doesn't fuse flour and butter, but allows it to form flakes. When you're using multiple racks, the food itself won't interfere with the heat that reaches other foods. For all these reasons, convection ovens are no longer reserved for high-end restaurants, but increasingly find themselves in the renovated kitchens of amateur cooks."