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 susceptable 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 around 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 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.