The good and the possibly bad.
Creatine is a naturally occurring substance that occurs in meats. It is converted to creatine phosphate and then to ATP-adrenotriphosphate????-I.E. energy which is used for cellular muscle contractions.
Some don't get enough in the diet and high level performance athletes burn it up fast. Thus some feel that supplementation is beneficial. It has some downsides in that long term use will probably cause a weight gain if you are not burning it off rapidly. The weight gain is probably mainly due to water retention.
I don't use it on a daily basis, but only if doing a hard workout or will be physically active for an extended period of time.
A quick overview of creatine.
http://www.webmd.com/men/creatine
