Chitin is one of the main components in the cell walls of fungi, the exoskeleton of insects and other arthropods, and in some other animals. It is a polysaccharide, made out of units of acetylglucosamine (more completely, N-acetyl-D-glucos-2-amine). These are linked together in β-1,4 fashion, the same as the glucose units that make up cellulose. So chitin may be thought of as cellulose, with one hydroxyl group on each monomer replaced by an acetylamino group. This allows for increased hydrogen bonding between adjacent polymers, giving the material increased strength.
"Chitin" and "chiton" (a marine animal) both derive from the same Greek word meaning "tunic", referring to the hardness.