Plywood is probably the most popular and versatile man-made woodworking material available in home centers today. Plywood is a laminated product made up of numerous thin strips of wood laid in alternating directions and bonded with glue into strong, stable sheets. Because of this construction method, plywood is less susceptible to expansion and shrinkage. Why?
A board of solid wood hewn from the trunk of a single tree is somewhat unstable and likely to expand or shrink across the grain based upon the moisture content present in the wood. While the board is much stronger and less likely to expand or shrink along the grain, it is much more likely to split with the grain than against.
Plywood's construction addresses both of these concerns. By gluing numerous thin strips of wood in alternating directions layer by layer, plywood is much less likely to expand or shrink based on moisture in the environment, and the alternating construction creates a board that is much stronger in each direction than a similarly sized board cut from a single tree.
Most plywood is made from an odd number of layers, called plies (typically 3, 5 or 7), with an equal number of plies sandwiched on each side of the center ply. In this manner, the surface plies are always parallel, and the grain of the surface plies usually follows the longest side of the sheet (although not always). The sheet is strongest when parallel with (as opposed to against) the two surface plies.
In most applications, one side of the plywood is more likely to be seen than the other side. As such, plywood is usually sold with one better side, called the face, and one side that isn't as clean or smooth called the back. Plywood with two faces is available for projects where both sides of the sheet will be visible.
Home centers and lumberyards often sell various types of hardwood plywood such as oak, birch, or cherry plywood. In reality, these sheets aren't made from entirely from that specific hardwood, but instead are veneers made with the same spruce, pine or fir (SPF) material as a regular plywood with the top layer only made out of the desired hardwood. Keep this fact in mind when sanding the surface of a hardwood plywood, as you'll want to avoid sanding through the thin hardwood layer, exposing the SPF layer beneath.
Uses for Plywood:
Plywood is a very popular material used in home building, particularly for its strength as a roofing underlayment. Other comparable but less expensive sheet goods, such as oriented strand board (OSB) or medium density fiberboard (MDF) may be a better choice for some specific choices. For instance, OSB is often used for exterior wall sheathing because it is inexpensive and provides good linear strength, but is not well-suited for weight-bearing installations. MDF is a good choice for painted projects and laminate-covered products and is commonly used with veneers for inexpensive furniture.
For fine woodworkers, hardwood and sanded plywoods are ideal choices for building cabinets. The durability and sheer strength of 3/4-inch plywood (to be clear, you'll find it at your home center listed as 23/32" plywood) combined with the hardwood veneer that can be stained to a beautiful finish makes it the material of choice for both base and wall cabinets in kitchens, utility rooms, garages, and more. While it isn't the best material for cabinet doors (routing a profile onto the edge of plywood will reveal the unsightly sandwich layers that make up the plywood), matching pieces of the same species of hardwood as the plywood can be stained to match the cabinet carcasses.