Sleet is snow that has partially melted as it falls through changing temperatures in the atmosphere. Temperatures may rise above freezing as a snowflake falls to the ground, causing it to lose its hexagonal shape and become sleet.
Very cold conditions in the upper atmosphere cause water vapor that normally turns to rain to freeze around dust particles. These frozen particles take on a hexagonal ice-crystal shape; this is what is defined as snow. For snow to reach the ground, atmospheric temperatures must remain below freezing as the snow falls. Changes in temperature result in snow partially melting and being reclassified as sleet.
Sleet is not to be confused with freezing rain. If snow completely melts then hits a layer of freezing air just before it strikes an object on the ground it may freeze instantly upon contact. This creates a layer of ice on the object, which is markedly different from the slush formed by sleet. Both forms of precipitation can cause similar havoc for winter drivers, as sleet can pile up and become treacherously icy. Additionally, since it still retains some of the properties of snow, sleet can reflect off a driver's headlights and make it difficult to see clearly.