Hey MoonGoat- HDRI is a special image format which has significantly more than the base 8 bits per channel which allows for a much greater range of illumination data. With standard 8 bit, you can only have 255, 255, 255 as the most powerful light be it a candle or the sun. This image is applied to a sphere in a luminace channel and is used to provide lighting data for the model/scene. In a program that can use this data as image based lighting, the image area corresponding to the sun will have much more drive than the image area corresponding to the candle. This is not to be confused with radiosity, sometimes called bounce lighting, which is a rendering technique which allows light to be reflected from various surfaces while still providing illumination at the reflected location but affeceted by the qualities of the relecting surfaces material properties. Hope this help a little!

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