MAGNETRON

Inside the microwave oven is a high powered voltage system and is ideally used to create microwave energy.  This works by stepping up AC line voltage to high voltage and then changed to an even higher DC voltage.  The DC power is then turned into RF energy, which cooks the food. 

Magnetron's Structure...

The magnetron tube sits at the center of the high-voltage system.  The 2450 MHz energy that is needed for a microwave is produced by the magnetron, a diode-type electron tube.  An ordinary electron tube has a grid and since the magnetron does not, it is classified as a diode.  The grid is made up of the space between the anode (plate) and the cathode.  The external structures may change from magnetron to magnetron but the basic internal set up will stay the same.

www.gallawa.com/microtech/magnetron.html