Microwaves is a misnomer. There's nothing "micro" about them, in the scientific sense, which says that micro means "one millionth". The wavelength range of this radiation is about 3.0 cm to 1.0 mm, with the energies between 10^-5eV and 0.001 eV.

As with infrared radiation, many molecules vibrate and rotate when exposed to microwaves. When a photon of wavelength 12.2 cm is incident on a water molecule, the molecule will rotate, since it is polar (has a positively-charged end and a negative-charged end) and wants to align itself with the incoming E&M field. What this means is that any matter containing many water molecules (for instance, food) will tend to heat up due to the rotating and flipping water inside the substance. A cup of water boils quite nicely in a microwave oven. Bread, on the other hand, will dry out since the water evaporates easily. A paper napkin will hardly heat up at all.

Besides ovens, we use microwaves for many kinds of communication. Most television satellites and dishes operate in the microwave region, and any kind of radar device is emitting and receiving microwave signals. Short-wave radio operators are using microwaves to talk to each other around the world, because these waves can travel much further than weak radio waves.