In fact different mediums can be used in the same painting. For example, watercolor and acrylics are often used together. Oil paints must be mixed with an oil-soluble product such as linseed oil. This allows you to easily modify the painting as you go along. The brushes must be washed in turpentine afterwards.
Watercolors are in some ways the opposite of oils.
They are mixed and applied with water instead of with oil, and many of the colors are transparent. Traditionally parts of a water color painting are left unpainted allowing the white of the paper to add to the brightness. Watercolors tend to have a mind of their own, and will often run together in unexpected ways. Acrylics fall somewhere in between oils and watercolors. Acrylics are waterbased, but they are not transparent like watercolors are.