Erosion is the displacement of solids (soil, mud, rock, and other particles) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). Although the processes may be simultaneous, erosion is to be distinguished from weathering, which is the decomposition of rock. Erosion is an intrinsic natural process, but in many places it is increased by human land use. Poor land use practices include deforestation, overgrazing, unmanaged construction activity and road or trail building. However, improved land use practices can limit erosion using techniques like terrace-building and tree planting.

A certain amount of erosion is natural and in fact healthy for the ecosystem. For example, gravels continually move downstream in watercourses. Too much erosion, however, can cause problems, clogging streams with gravel, filling reservoirs with sediment, reducing soil fertility and water quality.

Details: http://www.answers.com/topic/erosion