A storm that produces freezing rain and coats surfaces with ice is called an ice storm. It is a winter storm whose defining precipitation is freezing rain.
Ice storms form when snow or another frozen form of precipitation falls through a layer of above-freezing air and melts into rain. Near the ground, the rain passes through a shallow layer of air below 0°C. The droplets become supercooled: they remain liquid even though they are colder than freezing. When they strike roads, branches, power lines, or other cold surfaces, they freeze into a smooth glaze.
This process separates freezing rain from sleet. Sleet refreezes into ice pellets while still in the air, whereas freezing rain freezes only after contact with a surface. The accumulating glaze can make travel treacherous and add enough weight to snap tree limbs or bring down utility lines. In the United States, the National Weather Service commonly defines an ice storm as one producing at least 0.25 inch of ice accumulation on exposed surfaces.