These two terms are commonly used interchangeably, even though they are very different machines that work on different principles and serve different purposes. As you can see from the pictures below, windmills generally have more blades than modern day turbines. This is because windmills are used to do work, such as grind grain or pump water from a well. Windmills operate on the law of conservation of momentum.
The conservation of momentum can be easily understood on a pool table. The cue ball in motion has momentum and when it hits the other pool balls, the energy is transferred. The energy is not always transferred in a linear fashion however, as can be seen in the image at the right. Windmill blades are pitched so that when the wind hits them, it doesn’t push the blades backward but moves the blades rotationally.
The linear momentum of wind is transferred into rotational momentum as it hits the angled windmill blades, causing them to spin. That rotational energy then powers gears or shafts that then do work.
Comparatively, Wind Turbines produce electricity. The blades are connected to a low speed shaft which is connected to either a gearbox or generator. See the section How a Wind Turbine Works to learn more about generators. Another key difference between windmills and turbines is that turbine blades operate on the principles of lift and drag, similar to that of airplane wings. As wind passes over an airfoil shape, a high and low pressure area form, causing lift, or in the case of wind turbines, causes the blades to rotate. See the section How a Wind Turbine Works to learn more about the lift/drag principles.