What is it?
Sheet erosion is the planar removal of surface soil by the action of either raindrop splash, shallow flows of surface of water, or even by wind.
Rill erosion is usually linked with sheet (water driven) erosion as the shallow flows of water driving sheet erosion tend to coalesce and thus increase both in velocity and scouring capacity.
Additional information on the mechanisms of water erosion is given on the soil health site
Sheet erosion and rill erosion of a cropped paddock following an intense thunderstorm