A vast number of varieties of plywood exist for different applications. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir (collectively known as Spruce-pine-fir), and is typically used for construction and industrial purposes.
Hardwood plywood is used for some demanding end use. Birch 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. Birch plywood has excellent surface hardness, damage and wear resistance.
Decorative plywood is usually faced with hardwood, including red oak, birch, maple, lauan (Philippine mahogany) and a large number of other hardwoods.
Plywood for indoor use generally uses the less expensive urea-formaldehyde glue which has limited water resistance, while outdoor and marine grade plywood are designed to withstand rot, and use a water resistant phenol-formaldehyde glue to prevent delimitation and to retain strength in high humidity.
The most common varieties of softwood plywood come in three, five or seven plies with a metric dimension of 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1/10" through 1/6" depending on the panel thickness. Roofing can use the thinner 5/8-inch plywood. Subfloors are at least 3/4-inch depending on the distance between floor joists. Plywood for flooring applications is often tongue and grooved. The mating edge will have a "groove" notched into it to fit with the adjacent "tongue" that protrudes from the next board. This keeps the boards from slipping past each other providing a solid feeling floor when the joints do not lie over joists. Tongue & groove flooring plywood is typically 1" in thickness.
High-strength plywood, known as aircraft plywood, is made from mahogany and/or birch, and uses adhesives with increased resistance to heat and humidity. It was used for several World War II fighter aircraft, including the British-built Mosquito bomber which was nicknamed the wooden wonder.
Certain plywoods do not have alternating plies. These are designed for a specific purpose. One such plywood is known as "Bendy Board". This is very flexible and is designed for making curved parts.In the UK this is known as "Hatters Ply" as it was used to make gents stovepipe hats in Victorian times. However these may not be termed as plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain perpendicular on each layer.
Marine plywood is specially treated to resist rotting in a high-moisture environment. Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood; costs for a typical 4 foot by 8 foot 1/2 inch thick board is roughly $75 to $100 US or around $2.5 per square foot, which is about three times as expensive as standard plywood.
Other types of plywoods are fire retardant, moisture resistant, sign grade, pressure treated, and of course the hardwood and softwood plywoods. Each of these products are designed to fill a need in industry.
Source: Wikipedia
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