Creatine has stood the test of time as a sports supplement. More importantly, it has stood the test of hundreds of placebo-controlled clinical studies over the years.  

Creatine is synthesized in the liver from the amino acids arginine and glycine and stored in the major skeletal muscles including the heart. Once inside the muscle, creatine is phosphoylated to form creatine phosphate (CP) which is a high energy substrate that assists in the contraction of the myofibrils (muscle fibers). Creatine Phosphate is utilized to maintain higher levels of ATP (energy molecules) during exercise. Creatine Phosphate maximizes physical performance and reduces exercise fatigue by absorbing hydrogen ions released by muscles as lactic acid. Intense anaerobic exercise, such as weight lifting and sprinting, depletes ATP and greatly increases the demand for creatine.

Articles on Creatine:

http://en.wikipedia.org/wiki/Creatine_monohydrate

http://en.wikipedia.org/wiki/Creatine_phosphate

http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-creatine.html