Ice pellets (sleet in US English) form when snowflakes pass through a warm layer (>0 °C), partially or fully melting, then pass through a subfreezing layer near the surface where they refreeze into hard, transparent ice pellets.
They differ from freezing rain (which freezes on impact, not during fall) and hail (which forms inside cumulonimbus). Ice pellets produce a characteristic "tick-tick" sound hitting hard surfaces. They signal the presence of an upper-level temperature inversion.