The microwave generating component of a microwave oven is called a magnetron. The magnetron is actually a vacuum tube (a vacuum diode, actually) that has an associated set of magnets. This tube (the...
A microwave oven generates microwave radiation, which is a form of non-ionizing electromagnetic radiation. Links can be found below for more information.
Very basically, the oven shoots microwave radiation into the food. The reason why it makes the food hotter is because microwave radiation is absorbed into water, fat, and sugars directly, causing...