Glucosamine is a natural chemical compound in your body. But it also comes in the form of a supplement. 
Glucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides, chitosan, and chitin. Glucosamine is one of the most abundant monosaccharides. It is produced commercially by the hydrolysis of crustacean exoskeletons or, less commonly, by fermentation of a grain such as corn or wheat.
In supplement form, glucosamine is harvested from shells of shellfish or made in a lab. There are several forms of glucosamine, including glucosamine sulfate, glucosamine hydrochloride and N-acetyl glucosamine. These supplements are not considered interchangeable.
Glucosamine is vital for building cartilage. Cartilage is a flexible, tough connective tissue found in several parts of the body. This firm, rubbery tissue functions as padding at the ends of long bones where they meet joints.