Have you ever wondered what causes freezing rain?
Freezing rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are "supercooled," they instantly refreeze upon contact with anything that is at or below 0 degrees C (32 degrees F), creating a glaze of ice on the ground, trees, power lines, or other objects. A significant accumulation of freezing rain lasting several hours or more is called an ice storm.
For more information on how other winter weather precipitation types such as sleet or snow form, check out the excellent winter weather primer available from NOAA's National Severe Storms Laboratory.