A microwave oven and a convection oven seem to do the same basic thing: food goes in cold and comes out cooked. But fundamental differences in technology power microwave ovens and convection ovens. You don't need to understand complex physics principles or thermodynamics for the differences to matter: they have a real effect on how you can use the appliances.
Microwave and convection ovens generate and circulate heat differently. Convection ovens use fans to push heated air around the inside of the oven, heating the whole environment to cook food evenly. Microwave ovens generate radiation on the micro-wave end of the radioactive spectrum. This radiation excites molecules in the food, causing them to vibrate and create heat. Microwave ovens include turntables or mirrored walls to circulate the radiation evenly, effectively cooking the food in its own heat.
While convection ovens cook up to 25 percent faster than regular ovens, a microwave can cook up to 25 percent faster than a convection oven, depending on the food being cooked; foods with a high water or fat content cook faster. Convection/microwave combination ovens offer both methods simultaneously, for even faster cooking.
Microwave ovens don't use traditional heat to cook food, so they do not brown or broil them. A convection oven, like a traditional oven, cooks by applying high-temperature heat to the outside of the food. This means convection ovens can be used for browning or broiling. Food cooked in a microwave oven will more quickly cook all the way through, whereas convection-cooked food can be hot to the touch without being fully heated in the center.
Due to the molecular acceleration of microwave ovens, metals and some types of microwave-unsafe crockery cannot be used in them. Metal in a microwave oven will react electrically to the radiation, sparking and damaging the oven's inner workings. In convection ovens, objects without enough heat resistance can catch fire or melt. Do not place microwave-safe plastic or wood implements in a convection oven. As a rule, don't put anything that would quickly perish in a fire into a convection oven.