Hügelkultur is a composting process employing raised planting beds constructed on top of decaying wood debris and other compostable biomass plant materials. The process helps to improve soil fertility, water retention, and soil warming, thus benefiting plants grown on or near such mounds.
Complete info on the what, why and how of Hügelkultur beds: Rich Soil
The many benefits of Hügelkultur: Permaculture Magazine
The Art and Science of Making a Hügelkultur Bed: Permaculture News