By gluing numerous thin strips of veneers in alternating directions layer by layer, plywood is much less likely to expand or shrink in moisture environment, and this structure is much stronger than similarly sized lumber cut from a single tree.
Plywood is made from an odd number of layers, each layer called ply(typically 3, 5 or 7), with an equal number of plies on each side of the center ply. The plies face are parallel, and the grain of the surface plies follows the long side of the board(although not always). The sheet is the strongest parallel with (as opposed to against) the two surface plies.
