Plywood is graded using a system of letters from A to D.
A and B grade is the highest quality(also the most expensive) of plywood you can get. All of the knots in the wood are replaced with football shaped patches, which can be sanded and finishes with the rest of the panel. Also any gaps or voids along the edges of the panel are usually filled and smoothed off with wood putty.
C and D grades are the most economical types of plywood. You will see more repaired knots and voids on a sheet, and some defects will be left as is. This is the most common plywood found in the majority of home hardware stores.
Plywoods are then broken up into their own catergories:
Softwood plywood – A plywood usually made either of cedar, Douglas fir or spruce, pine, and fir wood. It is typically used for construction and industrial purposes.
Hardwood plywood – It is a plywood that is made out of wood from hardwood trees and used for demanding end uses. Hardwood plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Oriented plywood construction has a high wheel-carrying capacity. Hardwood plywood has excellent surface hardness, and damage- and wear-resistance.
Tropical Plywood – A plywood that is made of mixed species of tropical wood. Originally from the Asian region, it is now also manufactured in African and South American countries. Tropical plywood is superior to softwood plywood due to its density, strength, evenness of layers, and high quality. It is usually sold at a premium in many markets if manufactured with high standards. Tropical plywood is widely used in the UK, Japan, United States, Taiwan, Korea, Dubai, and other countries worldwide. It is the preferred choice for construction purposes in many regions due to its low cost. However, many countries’ forests have been over-harvested, including the Philippines, Malaysia and Indonesia, largely due to the demand for plywood production and export.
Aircraft Plywood – Is a high-strength plywood. Structural aircraft-grade plywood is more commonly manufactured from African mahogany or American birch veneers that are bonded together in a hot press over hardwood cores of basswood or poplar. Basswood is another type of aviation-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength.
Marine Plywood – It is a plywood that is manufactured from durable face and core veneers, with few defects so it performs longer in both humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. More recently, tropical producers have become dominant in the marine plywood market. Okoumé from Gabon is now the accepted standard for marine plywood, even though the wood is not very resistant to rot and decay. Each wood veneer will be from tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.