Sheet and rill erosion occurs on sloping land with little ground cover. When it rains water runs down the hill, and without plants to hold the soil in place, the flowing water washes the soil away. Sheet erosion is when the water removes even layers of soil. Rill erosion is when the water makes channels up to 30 cm deep.
Gully erosion is when water makes a deep channel that washes away soil when it rains. Each time it rains, the channels get deeper as more soil is removed. The soil can wash away into creeks and streams and block the water flow and discolour the water, or damage roads. The loss of topsoil reduces the amount of area available for farming.
Mass movement is when the erosion is helped by gravity, including landslides and avalanches. Mass movement not only removes a great deal of earth and rocks, it can destroy houses and farmland.
Wind erosion is when the wind lifts and removes topsoil. In dry areas in particular, soil that is not kept in place by plants is easily removed by the wind. Where crops have been grown repeatedly without giving the land a rest, the soil becomes less bound together and easily breaks down and is removed. Where animals have grazed too much or have trampled the earth hard, the plants are no longer holding the soil together, and it can be removed by the wind. The wind dumps the soil elsewhere, and can clog other farmland and roads.
Soil degradation is the term used to cover the damage that occurs to soil. Soil erosion is just one of those problems.
Source and More information available at: http://www.kidcyber.com.au/topics/erosion.HTM