Rain, wind and freezing are the causes of the erosion of a landscape.
Wind erodes high places via a mechanical effect. Particles are torn away as it passes. The polishing or abrasive power of the wind is reinforced by the presence of sand and dust.
Water has a mechanical effect (from the impact of water drops, known as the "splash effect"), and a chemical one (dissolution of calcareous substances, like limestone).
The durations of the effects of these phenomena are quite variable -- from a few hours (avalanches, hurricanes) to several millions of years.
Note finally that gravity, which pulls all matter downward, is an omnipresent phenomenon in the levelling of terrain.