Creatine is an organic acid used by the muscles and obtained in the diet from meat, and also synthesized in the human body from three amino acids, arginine, glycine, and methionine. Creatine obtained via the diet and biosynthesis are usually all the human body needs, but athletes training for strength and power can benefit from its use. Vegetarians and vegans gain the most benefit from supplementing with it; creatine supplementation tends to cause the greatest increase in intramuscular creatine levels for those whose creatine levels begin low.
Creatine in the body is phosphorylated to form phosphocreatine. Phosphocreatine reacts with ADP by donating its phosphate group, thereby producing ATP. Stored ATP is the primary energy source for short bursts of exertion, such as a short set of a particular lift or a brief sprint. Therefore, by replenishing stored ATP, creatine enables athletes to train harder in this regime; muscular fatigue is less limiting, and a set can be completed at a higher level of intensity or with more reps. Creatine has essentially no noticeable effect on exercise performed beyond this regime, however. It is therefore an appropriate supplement for strength athletes, bodybuilders, sprinters, and other athletes in events where short bursts of strength or power are needed, as well as athletes who perform general strength training to aid in their sport.
 Bodybuilding woo
Creatine has an important side effect that causes some in bodybuilding woo circles to overestimate its importance. Supplementation with creatine causes water retention, because most forms of consumed creatine, like creatine monohydrate and creatine ethyl ester, tend to carry a "shell" of water around them. Therefore, creatine in the muscles can flush them with water, causing the muscles to look "pumped" and generally bigger. This apparent gain of muscle mass immediately after beginning creatine supplementation can be mistaken for hypertrophy.
Worse yet, creatine that does not make it into the muscles can cause water to be retained subcutaneously. This produces the characteristic "creatine bloat", a puffy appearance that gives the illusion of increased body fat, often first noticeable via an increase in waist size. Unnecessarily high doses of creatine are more likely to lead to this bloat, since there is a limit to how quickly creatine can be taken into the muscles. Furthermore, the water retention associated with creatine can cause dehydration.
There is some question whether creatine supplementation in large amounts is safe. Creatine in the amounts normally obtained through human nutrition is safe, and indeed used in the body. Furthermore, no side effects associated with fairly high doses (20g per day) of creatine have been found. However, taking creatine in larger doses may stress the kidneys; creatine turns into creatinine, which is toxic. It might be contraindicated for people with poor kidney function.
- Quackwatch has a more extensive article on the benefits, myths, and possible adverse effects of creatine supplementation.