Weathering is the natural process by which rocks are broken up into smaller fragments or chemical elements. Weathering can be chemical or physical; both require certain circumstances and both can happen at once. For example, pebbles and sand on a beach have undergone both physical weathering -- rubbing against each other reduces them in size -- and chemical weathering -- reacting with the chemical properties in the ocean water.
In a warm, wet climate, the weathering of rocks will be more chemical, as the higher degree of precipitation (rain) acts on the rocks in a chemical sense instead of a physical sense. The chemicals contained in rainwater react with the chemical composition of the rocks, causing them to change; this can cause new compounds to form, or cause parts of the rocks to erode. In a cold climate, water seeping into cracks will freeze and expand, breaking the rocks; this would be physical weathering. In warm climates, the water will run off the rocks and carry small parts of the rocks away as the chemicals react.
Water with a high mineral content can cause chemical buildup; this is seen in cave systems with stalagmites and stalactites. However, rain water does not contain a high concentration of minerals, and so does not cause buildup; in fact, rain water is more likely to be acidic, thus causing even more pronounced chemical reactions.