Windmills are rather simple machines powered by wind energy. Wind turbines make use of wind to turn rotating blades. In earlier times, windmills were used to produce energy to grind grain and pump water. Nowadays, windmills are used to turn wind energy into usable (electricity) energy by making use of its rotating blades.
Several years ago I bought some remote property in Arizona. I am an astronomer and wanted a place to practice my hobby far away from the sky-wrecking light pollution found near cities of any real size. I found a great piece of property. The problem is, it's so remote that there is no electric service available.