Sawdust consists of fine particles of wood. It is produced from cutting with a saw, and is a by-product of manufacturing industries. Several uses include its use as a fuel.

Wikipedia definition:

Sawdust or wood dust is a by-product of cutting, grinding, drilling, sanding, or otherwise pulverizing wood or any other material with a saw or other tool; it is composed of fine particles of wood. It is also the byproduct of certain animals, birds and insects which live in wood, such as the woodpecker and carpenter ant. It can present a hazard in manufacturing industries, especially in terms of its flammability. Sawdust is the main component of particleboard.

Source: Wikipedia - Sawdust