What are compost piles?

A hole in the ground. You put kitchen food scraps in there, and leaves and dead flowers from the garden. When it's full, you cover it. It turns into good earth. A few months later you dig it out and spread the earth, or compost, over your garden.

In a commercial operation: It is consisted of cement walls with a compost turner that is on tracks that run down the walls and turns the compost in between the walls.
A hole which is dug in the ground to accommodate carbon- and nitrogen-rich recyclables for natural breakdown into dark-colored, fresh-smelling, nutrient-rich organic matter is what a compost pit is. The hole may or may not be covered over with a thin layer of soil.