Plywood is a manufactured wood which is made by gluing several thin sheets of wood together with the grain alternately running along the sheet then across it. Some plywoods also have diagonal plies. Plywood is strong, light, and cheap. There are many different kinds, made by different methods with different types of hardwood, softwood, and glue.
The veneer is usually about 3 millimetres thick for each layer. Interior layers of these boards are usually made from an inexpensive wood while the outer veneer layers are made from more expensive timber to give the board a nice wood grain appearance. The glued layers are heated together to make the seal become better bonded.
A common reason for using plywood instead of plain wood is its resistance to cracking, shrinking and changing shape. It is used to make a range of things from sheds, to cladding to cabinets. It is graded on the quality, from class A to class D, A being the best quality and D being the worst quality.
Different kinds of plywood are made for different purposes. There is a huge world-wide industry making plywood for structural purposes. IKEA is well-known for using mostly plywood and chipboard in their indoor and outdoor furniture products.