Microwave ovens work by heating foods with a form of electromagnetic radiation. This radiation is reflected by metal, so it is generally safe, even when used near food storage.
Many people are concerned about microwaves because of the fear generated by the word "radiation," but microwave ovens do not emit radiation unless they are damaged and have leakage--in which case they shouldn't be used.
Radiation includes all forms of energy in the electromagnetic spectrum, including heat and sunlight. Some forms of nuclear radiation are very dangerous, but these are not used in microwaves.
Your refrigerator and your microwave are both large draws on electrical power when in operation. You don't need to worry about their physical placement, but be sure the electrical circuit you're using is rated to power both simultaneously.
Microwave ovens provide safe and quick heating of foods; it's no less safe on top of a fridge than it is on a kitchen counter.
Microwaves operate on the same frequency as home wireless networks. In many cases, using a microwave will temporarily shut down computer connections within a few yards of the microwave.