Whereas hail forms within clouds, sleet begins as snow falling to earth after having left its originating cloud. The snow partially melts on its trip down in a layer of warm air but then refreezes into ice crystals that appear much like hail.
Sleet is also used colloquially for rain mixed with some frozen droplets.
In the British Isles, sleet refers to precipitation which is a mixture of snow and rain.
See also freezing rain, hail, ice storm, wintry showers
