Erosion is the steady wearing away of the earth's surface. This is done with rain, wind and sand. An example of chemical erosion is water running on rock with iron in it. The iron turns into iron oxide. 

Ice erodes rock by melting and running in between cracks. It then refreezes and the rock breaks apart. Erosion is happening constantly and has been going on for millions of years. Mountains erode about one centimeter per year. 

Roots of trees and plants slow down and prevent erosion.
Source: Bill Nye The Science Guy by Disney