A grindstone is usually curved out of a thick, porous stone such as sandstone and given a fine grain. The stone is mounted onto a central axis allowing the grindstone to be spun at varying speeds. With the grinding process, items such as knives and tools can be sharpened; however it can also be used to polish wood or stone (the fine grain of the grindstone will buff rather than gouge).
Wikipedia article on grindstones.
