Microwaves are very-high-frequency radio waves that swing back and forth at a frequency of about 2 billion cycles per second. During this process, they make certain molecules move, and once they're moving, they're hot. Microwaves enter food from the outside, and penetrate instantly into a chunk of food, heating and cooking as they go.
Source: Excerpted from FDA/CFSAN Food Safety A to Z Reference Guide, September 2001