Here's how it works: http://home.howstuffworks.com/microwave.htm
The magnetron tube inside the oven produces microwaves. The waves enter the oven through an opening in the oven cavity. The microwaves move in a straight line — not a good recipe for evenly cooking food. Many ovens use turntables to rotate the food itself or stirrer fans to distribute the waves evenly. Food is cooked from the outside to the inside, as in regular cooking.
When buying a microwave, consider:
To determine the size and power of the oven, think of the number of people in your family and whether you plan to use the microwave for primary cooking. Selecting a microwave oven for cooking full meals for a family will require a larger, high-power unit.
Also consider the size of your cookware. Will it fit in the oven? How will you use the microwave? For more advanced cooking, look for defrost/cook/keep warm options.
How much room do you have for the microwave? Ovens are available as countertop, under-the-cabinet, over-the-range, or built-ins.
Capacity and Power
Compact (less than 0.8 cubic feet)
500 — 800 watts
Midsize (0.8 to 1.2 cubic feet)
800 — 1000 watts
Full-size (1.2 cubic feet and larger)
More than 1000 watts
Remember that cook times offered in recipes will vary based on the wattage of the microwave.
Turntables provide even cooking. Removable turntables won't limit the size of the cooking dish used as much as a fixed turntable.
Convection combines radiant heat with microwaves to speed up cooking and provide crusty baked goods and juicier roasted meat.
Browning units act as the broiler in a conventional oven would.
Probes or Sensors allow the oven to gauge whether the dish is cooked thoroughly and prevent overcooking.
Timers on some models can be used in the normal countdown function or come preset to certain types of foods (for example, popcorn).