The choice between block board or plywood depends on the project. If you want a smooth finished surface and the strength of the panel is not a major factor, block board makes a good choice. But if strength is more important, plywood is a better choice. For example, plywood can be used to lay down a subfloor for tile and engineered hardwood, while block board is unsuitable and would crush under foot traffic.
Block board is a piece of wood with a plywood core and thin veneer panels on the top and bottom sides. The plywood core is made of solid wood strips, and a finished block board panel typically ranges in depth from about 3/8-inch to 1 3/16-inches. Block board is made for interior use, only, because the wood strips are glued with interior glues and the layers are hot-pressed to form a solid panel. As a result, if block board is used outdoors, the layers will, over time, delaminate and become unglued. Block board is often used for furniture pieces such as desks and tabletops.
Plywood is one of the most versatile wood materials available. It is composed of thin sheets of wood which are glued together under pressure. Plywood is used for interior purposes such as flooring and making furniture, and for exterior purposes such as sheathing a roof. Plywood is graded with two letters from A to D, based on the appearance of the front and back panels. For example, a panel with an "AC" grade has a smooth front side and minimal knots on the front, representing the A-grade, while the C-grade indicates that the back side has visible knots and imperfections.
Plywood and block board are both sold in panels and readily available at lumber and home improvement stores. You can use either one for interior projects such as furniture and cabinets. However, they are not pliable materials and cannot be used for applications that require a curved edge, for example, or one that requires routing to create grooves. Plywood and block board are only suitable to use for a flat surface such as a cabinet door or table surface. You can also prime and paint them or add a veneer laminate to the surface if you want a wood-finish look for your project. Both are easy to saw and cut with common tools, such as a circular saw or jig saw. They also take to adhesives well, including contact cement and liquid nail products.
A plywood panel displays the grain characteristics of the wood while a block board panel does not display grain. Therefore, if you use block board and want it to display a grain, you need to apply a veneer laminate sheet to one or both sides of the panel. Block board has a smooth surface so you don't have to sand it. If you want to avoid sanding, you need to use plywood with an A-grade on the side of the panel that will be showing. Block board is not graded.