How did we ever live without microwave ovens? Bill, the Engineer Guy, provides perhaps the best explanation on the web of how these wonderful machines work.
A microwave oven contains three main components: a vacuum tube called a magnetron, which generates the energy to heat the food; a waveguide in the oven wall that directs energy to the food; and a chamber to hold the food and safely contain the microwave radiation. Food is filled with water. The water molecules are positively charged at one end and negatively charged at the other. These molecules are exposed to magnetron’s electromagnetic waves, which are made up of electrical and magnetic fields that change direction rapidly. The water molecules will try to align with the radiation’s electric field. As the changing field rocks the molecules back and forth rapidly, heat is created by this molecular friction.