You often hear that microwave ovens cook food "from the inside out." What does that mean? Here's an explanation to help make sense of microwave cooking.
Let's say you want to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F (177 degrees C) or so, but this time you accidentally set the oven at 600 degrees F (316 degrees C) instead of 350. What is going to happen? The outside of the cake will burn before the inside even gets warm.
In a conventional oven, the heat has to migrate (by conduction) from the outside of the food toward the middle (see How a Thermos Works for a good explanation of conduction and other heat transfer processes). Hot, dry air on the outside of the food evaporates moisture, so the outside can be crispy and brown (for example, bread forms a crust) while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There's heat everywhere all at once because the molecules are all excited together. There are limits, of course. Microwaves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."
In a microwave oven, the air in the oven is at room temperature, so there's no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.
For more information on microwave cooking and related topics, check out the links below.