Although both of these ovens use convection to carry heat, only one of them has the word "convection" in its name. The difference between them is that while a standard oven waits for natural convection to move heat slowly from the heat source to the food, a convection oven uses a fan to drive heat rapidly from source to food. While natural may sound appealing, a convection oven is simply better than a standard oven. Here's why:
In natural convection, hot air's buoyancy carries it upward through cooler air. Hot air is less dense than cool air and it floats upward the same way a bubble floats upward in water. With the heat source at the bottom of the standard oven, warmed air floats upward to touch and heat the food above it. But the airflow pattern in a standard oven is erratic, unpredictable, and easily upset by the very food that it's trying to cook. If you overfill the oven, the food won't cook properly. And even when the oven is almost empty, heat is transferred more efficiently to the bottom of the food than to its top so it's easy to burn the bottom.
But in a convection oven, hot air is stirred rapidly throughout the oven. Heat flows evenly into the food from all sides, leading to speedy and balanced cooking. The tops of foods brown and crisp nicely and the bottoms rarely burn. If it weren't for the added expense of the fan and the modest space the fan takes up inside the oven, convection ovens would be the new standard ovens.
Answered by Louis A. Bloomfield of the University of Virginia