Usually made of wood waste (sawdust), but you can make it from just about anything with cellulose: jute, bamboo, rice, wheat, sugar cane, flax. Often used in construction for walls bracing and flooring. Reasonably priced home furniture is often made of particleboard (often hidden under a wood or laminate veneer)-- it's lightweight, sturdy, and less expensive than solid wood furniture (And, as IKEA has demonstrated, does not have to be ugly).