Erosion is the relocation of sediment, be it weathered rocks and minerals, organic materials, or even dissolved material, from one place to another to be re-deposited elsewhere as a new sediment deposit. Erosion can be accomplished via wind, ice or water and is responsible for the reshaping of much of the exposed landscape.
Water erosion is the most prominent form of erosion in terms of volume of sediment moved, and the process is an important part of the formation of many landforms we live near. Canyons, cliffs, waterfalls, natural bridges, even river dunes are all made in large part by water erosion.
You can see how water erosion works in this easy experiment.