A waterbrush (Buy Direct) is unlike any other brush. It consists of a familiar-looking bunch of bristles at the one end, but the handle isn't solid wood or plastic. Rather it's a container or reservoir designed to hold water. The two bits screw together, and the clip-on cap stops the water leaking out when you're not using the brush.
As you use the waterbrush, water gradually seeps down from the reservoir onto the bristles. This means the brush bristles are permanently moist or damp.
Different brands of waterbrush look more or less the same, and all work on the same principle. The size and shape of the water reservoir differs between brands, as can the size of the brush bristles.