Weathering is more generally an exposure of a natural formation to the elements around it. Therefore, weathering can be chemical or mechanical. Mechanical weathering only changes the shape of a formation, while chemical weathering changes the actual chemical makeup of a formation.
Either type of weathering weakens a formation, causing it to easily break down when exposed to outside elements such as wind, snow or rain. Therefore, since the formation has already been 'weathered' or weakened, when natural elements then occur around it, it is more likely to erode and break down.
Erosion happens mainly as a result of weathering – the effect of water, temperature and wind on the landscape. Water causes much erosion. When it falls as acid rain, it can dissolve rocks that are sensitive to acid. Marble & limestone weather when exposed to the rain. When the rain falls very heavily, as in monsoons, then flooding can happen.
Rivers with a lot or rushing water can cause mud slides and erode river banks.
The action of waves on a beach causes much erosion. The waves pound on the rocks & over time, cliffs crumble. That is why you will often find sand & little pebbles on beaches. Rushing water, like what you find in rivers that move quickly in the mountains or strong waves on the shores of oceans, roll rocks around. This causes the sharp edges of the rocks to get knocked off & that is why river rocks are so smooth & beach pebbles look polished.