Cooking is all about heat and how heat transforms things. The way that you get this heat into the food differs, though. A frying pan uses hot metal and oil, while a broiler uses the direct, radiant heat of burning gas. Ovens use air to transfer heat: you burn gas or heat an element at the bottom of the oven, and the hot air rises up into the oven, cooking your food.
That works pretty well most of the time, but there is one problem: hot air rises. That's pretty obvious when you open the oven door, as the hot air rushes out and escapes toward the ceiling of your kitchen. What might be less obvious is that a similar thing happens when the oven door is closed, as the hot air that cooks your food rises to the top of the oven. This makes this top of the oven much hotter than the bottom.
In a typical oven, this difference can be very large. I tested my own (rather old) gas oven, and found that when the oven was preheated to 300 F, the top rack was at a toasty 330 F, while the bottom rack was at a much cooler 300 F. That's enough to make a huge difference in how quickly (and how well) things cook. See the example below, cooked in a Frigidare electric range by our intrepid reviewer Brian Bennett.
The top tray of biscuits (on the left) looks nicely browned, but the bottom tray isn't cooked properly. If you were making a lot of biscuits, you would need to either just use one tray, or try and keep track of the different cooking times for top and bottom trays. That's not very efficient either way.
So how do you make an oven more efficient? You get things moving. Convection ovens add a fan that circulates the air (sort of like an) inside the oven, pulling air out from one area (usually at the rear) and pushing it back into the oven, usually at the top of the cooking space. This circulation creates a more even temperature throughout the space, and also brings more hot air into contact with the food, which means quicker cooking.
For example, look at these biscuits cooked in a high-end Dacor convection oven by Ry Crist: both trays of biscuits are cooked to the same golden-brown color.
Some high-end ovens also add an extra heating element alongside the fan so that the air is heated as it circulates. This technique is often called true convection, European convection or sometimes third-element convection. Other models offer different convection modes, varying the speed of the fan and the heating to control the flow of air inside the oven. A convection oven can also work as a normal oven: if you just turn the fan off, it works normally.
The pros of convection ovens
The main pro of convection ovens is quicker cooking. The increased air flow means that more heat gets into your food, so it cooks quicker and more evenly. For most foods, the cooking time falls by about 25 percent, which is a significant decrease if you have several dishes to cook. Some ovens can adapt cooking times for you, automatically calculating the shorter cooking time when you turn convection on. This is not a given, though, so watch out when setting a timer.
Some chefs also think that convection cooking is better for roasting, as the circulating hot air heats the surface of the food, making the skin crispier and dryer on meats.
The other big plus of convection ovens is that the temperature is more even through the oven, meaning that you can put more food in there. Temperature-sensitive foods like breads or cookies can benefit hugely from this; a decent convection oven could handle two or three trays of cookies at once, cooking them at the same speed. Try that in a conventional oven and you'll get burned cookies.
Shorter cooking times also mean that you use less energy, so a convection oven can save money. This makes them particularly beloved in commercial kitchens, where a combination of shorter cooking and more even temperatures means you can cook things quicker, and cook different things at once.
The Cons of Convection Ovens
Some foods don't appreciate convection cooking, though. Foods like souffles and cakes don't rise as well, as the air flow can break down the delicate structure of the foods as it rises, and nobody wants a flat souffle or a light angel cake. Even food that you would not think of as delicate can have this issue: if you cook muffins in a convection oven, the tops may come out lopsided, because the delicate top was pushed over by the circulating air. That's why most convection ovens allow you to control the convection feature, slowing down the flow of air or even disabling it completely.
So, if convection ovens are better than normal ones, why aren't all ovens convection? It's a question of cost. Adding fans and extra heaters to the oven makes it more complex, and thus more difficult (and expensive) to make. GE, for instance, has its cheapest free-standing gas range priced at $550, but the comparable convection oven costs $900.
So, a convection oven can cook faster and more evenly, but at a higher cost. Like most examples of appliance science, the choice comes down to which is more important to you: money or time. And that is a question that no scientist can answer for you.