Cement board is a building material primarily used as a substrate for tile, stone, and other finishes in wet areas like bathrooms, kitchens, and exterior walls. It is made from a mixture of cement and reinforcing fibers, typically fiberglass, which gives it strength, moisture resistance, and durability. Cement board is known for its ability to withstand exposure to water, mold, and mildew, making it an ideal choice for areas that are prone to moisture.