Here is a quote from this great article I found on Gardening Channel:
"Is your backyard too hot and dry to cultivate the vegetables you have only dreamed of? Keyhole gardens were developed for the sole purpose of maximum crop output in the hottest and driest of conditions. Their low cost, low maintenance, and versatility make them a desirable gardening option for your yard and for gardening across the globe. ... Keyhole gardens are circular raised bed gardens. The larger outer circles are where crops are planted. The center portion of these gardens are active composting baskets. Small aisles are built to access the compost baskets. Keyhole gardens get their name from the bird’s eye view of these features.
The composting basket is center to the success of keyhole gardens. Kitchen scraps and grey water are added to the compost basket daily for continuous replenishment of the soil. The soil for keyhole gardens is specially layered to boost its ability to maintain moisture and nourishment."