Although specialty plywood can have any number of plies above three, most plywood is categorized as 3-ply, 5-ply or multi-ply. 3-ply is one of the most common types of plywood. It is about 2- to 3-millimeters thick and is generally used indoors, since it looks more decorative than thicker plywood boards. The most prominent use of plywood is in structural applications. Plywood can naturally withstand a lot of stress and full weather exposure.