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 cheaper and stronger than most plain woods.
Different kinds of plywood are made for different purposes. Marine plywood, for example, is especially resistant to water. There is a 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.