In one way, sleet and freezing rain are the same – nobody really wants to see either outside the window (or coming down on their heads). The key difference between sleet and rain, however, is that freezing rain is extremely cold but does not freeze until it falls on the ground, whereas sleet consists of raindrops which have already frozen on their way down to the surface.
Frustratingly, sleet actually has two different meanings, depending on where one is in the English-speaking world. In the British and Canadian use of the term, sleet actually refers to mixed rain and snow, or slush. In American parlance, however, sleet refers to a form of precipitation more similar to hail: small frozen pellets of ice which bounce off the ground. Once on the ground, these pellets can accumulate in much the same way as snow, although they are heavier and therefore more time-consuming to shovel.
Sleet forms slightly over one mile above the surface, when falling snow passes through a warm band of air. The snow has formed in the cold upper layer, but melts as it passes through the warm layer. If the warm layer extends to the surface, then this precipitation will continue to fall as simply rain. However, if the warm band is shallow, then the melted precipitation will re-freeze on its way to the surface, becoming sleet.
Freezing rain, or “ice storms,” occur in somewhat similar conditions, but are much more serious threats to human safety. In this case, what is falling is actually rain; however, it is very cold, and freezes extremely quickly upon hitting the surface. The result, often, is roads, power lines, and structures coated with a thick layer of ice. Results include difficult-to-spot ice patches on roads, and power lines which collapse under the added weight.
Recall that sleet forms when falling snow passes through a warm layer, melts, and then re-freezes on its way to the ground. Freezing rain forms under similar conditions, except that the warm layer is much wider and/or closer to the ground. The falling snow melts in this warm layer, and has been cooling – but not sufficient to freeze – when it strikes the ground. Such drops are often supercooled – that is, they have cooled to a temperature somewhere below the freezing point of water, but have not yet actually frozen. In either case, such drops freeze either instantly upon striking the ground, or very shortly thereafter, creating a dangerous layer of ice.
Whereas sleet accumulates as separate pellets similar to snow, freezing rain accumulates together on the ground, forming coherent masses of ice which cannot simply be shoveled away, but must be melted or covered over for the sake of safety.