If you live on the downwind side of a large lake, you're probably all too familiar with this weather phenomenon. It happens when cold winter air moves over a relatively warm body of water. What you get are small-scale but intense snowstorms.
Lake-effect snow occurs on such a small scale, almost on the scale of a summertime thunderstorm. One portion of a neighborhood or city might be under heavy snow, where a few miles away you may be under sunny skies.