Weathering refers to physical changes in landforms. If water gets into a crack in a rock and then freezes, for example, the water will expand, causing the crack to get larger. If this happens often, the crack will widen enough to break the rock into pieces. This can affect individual rocks or huge mountains.
Erosion involves movement. When moving water has sufficient depth and speed, it carries dirt, rock, and other objects along with it. As sediment is carried away from one location by that moving water, the original landform is being reduced or reshaped in size. The classic example of erosion is the Colorado River creating the Grand Canyon.
Deposition is the deposit of sediment in a new location. When moving water slows in speed, whatever was being carried by the water sinks to the bottom and is then compressed by the weight of more matter deposited on top of it. This process creates new land at the mouths of rivers, for example. Consider the delta at the mouth of the Mississippi River where it enters the Gulf of Mexico.