Creatine, chemical name methyl guanidine-acetic acid [NH2-C(NH)-NCH2(COOH)-CH3], is a nitrogenous organic acid naturally occuring in vertebrates that helps to supply energy to muscle cells. Creatine derives from the Greek word kreas (meaning meat) and is a combination of three amino acids - arginine, glycine, and methionine.

In the muscles, a fraction of the total creatine binds to phosphate - forming creatine phosphate. The reaction is catalysed by creatine kinase, and the result is phosphocreatine (PCr). Phosphocreatine binds with adenosine diphosphate to convert it back to ATP (adenosine triphosphate), an important cellular energy source.

Typically, half of the creatine (Cr) in our body comes from the food we eat (mainly from meat and fish), while the other half is synthesized from certain amino acids (glycine and arginine) in the liver, pancreas, and kidneys. 95% is stored in the skeletal muscles, the rest in the brain, heart, and testes.
Definition from Wikipedia.org.