Most old ones are watermills. They typically have a waterwheel and from 1 to 3 stones for grinding. Some have more. Water pours on to the wheel causing it to spin. This movement causes an axle inside to turn. The axle is connected to a gear which causes the grinding stones to spin. At the top, there is a chute filled with grain. The grain slowly falls through into the grinding stones.
There are different ways to transmit power from the waterwheel to the stones. The simplest method is known as a single-geared drive. Only one set of gears is used to drive a single stone. This arrangement was used since vertical waterwheels were used in gristmills. It was used until the 19th century.
Another method was known as spur-wheel drive. This method used a set of bevelled gears that would drive a horizontal 'spur-wheel', a large gearwheel that would then drive up to 3 or 4 millstones.
Another arrangement was known as a layshaft drive. The waterwheel would drive a horizontal 'layshaft' with gears mounted on it. Each gear would drive one set of stones.