Meteorologically speaking, rain is pretty simple: it’s falling water. So, too, is snow, which is frozen falling water and forms small flakes. There are several forms that cross between these two, however, that may cause a bit of confusion, and one of those things is freezing rain.
Like other cold weather forms of precipitation (sleet, hail and snow), freezing rain only occurs when the air is relatively cool. What happens when freezing rain falls, however, is quite different from these other forms, and potentially more dangerous.
Freezing rain is generated in the same method of the other forms. Water droplets are carried into the troposphere, forming clouds filled with water and ice crystals, though in the case of freezing rain a warm front has pushed into the region, floated above the cold air carrying the precipitation and warmed the atmosphere enough that the precipitation falls as rain. Not terribly warm rain, but rain nevertheless.
Because the warm front is so far from the ground, however, it has no effect on the ground far below, which typically remains rather cold thanks to Autumn or Winter conditions. As the precipitation falls the temperature begins to drop, and when the water hits the cold surface of the Earth it almost instantly turns back into ice. A single drop creates a relatively small patch of ice on a surface; millions of similar drops can turn wide territories into massive, ice-coated realms.
It’s this distinction that separates freezing rain from sleet, now or hail, all of which are at least partially frozen when they’re coming down. The impact of freezing rain is not what causes damage, it’s the aftermath of that impact. This definition is easy to confuse with sleet in particular, a mixture of water and ice falling simultaneously which can have a similar effect but with much less dangerous consequences.
And indeed, freezing rain can be deadlier than any amount of snow or sleet if it comes down in significant amounts. The ice is capable of quickly getting into cracks and turning large surfaces into flat, even ice rinks, making it very difficult to walk outside after a freezing rain storm. Freezing rain is particularly dangerous to pilots and their planes, as the rain can badly warp metal and freeze wings within minutes. Planes are routinely delayed for de-icing as a result of freezing rain. Freezing rain also proves a deadly foe to power lines, which with enough ice accumulation will sag and collapse, damaging the lines and potentially any property below.
Lloyd, Julie. A Pocket Guide to Weather. 2007.
Wikipedia: Freezing Rain