Microwave ovens use radiowaves at a frequency around 2,45 GHz to induce rotation in certain molecules in the food, water is the most affected in foods, though other dipole molecules, like fats and sugars will also be affected. It is the interactions between the moving molecules that create the heat(compairing it to every day experiences with friction is sometimes used as an aid to understanding it). The reason it heat the food quicker than an conventional oven is simply that it doesn't rely on heating air or using heat conduction, but dumps quite a bit of energy directly into the food.

Well, there are two effects that can cause uneven heating, one is if the microwaves doesn't penetrate to the center of the food, that might happen if something thick and absorbing is cooked. The thing would have to be several cm thick though, you would have an attenuation of 3dB a every 2-3cm in meat and liquid water, while dry foods or fat would have a lower attenuation, at least, that is if IIRC...

The other effect is that the microwaves tend to set up standing waves inside the oven, so that there are areas where destructive interference prevent heating. This last is the reason many ovens have a turning table or plate inside.

Setting the microwave at a lower setting can help too, as this can allow time for heat to spread more from hot areas to cooler areas.

Well, as I see it, since the primary source of heat is from the water, you would hardly get a temperature high enough to burn in most foods, unless it started to dry out... However you can get superheated water or steam, so things can get hotter than 100C...

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