The Wikipedia reference gives a fairly good description of fibre cement materials. Some are water resistant, some are waterproof...that is the board itself. Of course, there are joints, and there are fasteners. These are the weak points. The best advice is to follow the manufacturer's written instructions to the letter, whether the material is used as a tile underlayment, for exterior cladding, or whatever. I think Hardie is the largest manufacturer, but there are others.
