Sleet is the
precipitation (or
raining) of very fine, half-frozen bits of ice. Kind of like falling
slush. When raindrops fall from the sky, they pass through several different layers of air, each at a different temperature. If certain conditions are met and the temperature is just right, the raindrop will only half-freeze and turn into sleet. What usually occurs (especially in the summer months) is the cold layer of air turns the water droplet into
supercooled water, or
freezing rain. Alternatively, sleet sometimes becomes
hail. Sleet usually falls mixed with
rain or with
snow, and occurs in borderline freezing temperatures, such as early
Spring or late
Fall.
Sleet occasionally (not often) is created by a falling
snowflake melting in a warm layer of air, and re-freezing again before the snowflake is completely melted.
Sleet can leave an icy coating on cool exposed surfaces, such as metal (your car); however,
freezing rain is more often associated with this and is much more dangerous - especially to
aircraft.