A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's activity as a catalyst, a substance that increases the rate of a chemical reaction. Cofactors can be considered "helper molecules" that assist in biochemical transformations. The rates at which these happen are characterized by in an area of study called enzyme kinetics.

Cofactors can be divided into two types, either inorganic ions or complex organic molecules called coenzymes. Coenzymes are mostly derived from vitamins and other organic essential nutrients in small amounts.

Both prosthetic groups and cosubstrates have the same function, which is to facilitate the reaction of enzymes and protein.

More Info: en.wikipedia.org