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. However, if you mix in lots of water acrylics tend to act more like watercolors. I have done many paintings using this versatile medium. We are just starting to build our Web gallery of my paintings. Most of them are too large to fit on our optical scanner directly, so they will have to be photographed first.