A microwave oven is a kitchen device used for heating food using microwaves. Unlike conventional appliances which use heating elements, this works by irradiating food with high frequency electromagnetic radiation which causes water molecules in food to rapidly flip back and forth, attempting to magnetically align with the EM wave. All water molecules should be affected the same way within a microwave oven, as long as the EM wave is fluctuating through them. This results in friction, which causes the production of heat energy throughout the food. However, as there are parts in every wave which remain constant (as a sort of pivot point) this affect is not even. To help deal with this problem, most modern microwave ovens have a rotating platter on which food is placed, so the food moves through these hot and cold spots resulting in more even heating.
Microwave ovens are basically constructed as an EM resistant metal box (with a metal mesh over the front window act as a Faraday cage). Within this box, a step-up transformer feeds high voltage power into a magnetron, which generates an EM field in two directions. A waveguide then directs one half of this emission over into the food holding area where it interacts with water. The other half emission is typically reflected into a dead-end, so that it does not cancel out the waves in use from the other side, or cause any damage.