How does erosion occur?

When the elements wear on a certain object over time (water, wind, ice, fire). Take smooth rocks in a river for example. They are no longer ridged, but smooth due to the erosion from the water rubbing against it for long periods of time.

Basically, something rubs against something else for a long time, and then it wears the object down. Such as, it there were waves running over one of those huge rocks at the beach (in the water) the water would eventually erode the rock, making holes in places.