Creatine is one of the most misunderstood supplements of all time. A victim of a negative media spin, many believe that creatine is a dangerous and steroid like supplement that carries serious adverse side effects. This couldn't be further from the truth; it is one of the most thoroughly studied supplements of all time. In as simple explanation, the energy that fuels muscles is called ATP. Creatine simply provides your body with more of this substance. This in turn will help allow you to complete a few more repetitions than you normally would, aiding in muscle growth.
Rest assured, creatine doesn't affect hormone levels, so it isn't related to steroids by any means. Also, it is commonly said that creatine causes kidney damage, but this is a dehydration related side effect. The key with creatine (and any other supplement for that matter) is that you need to stay properly hydrated. Proper hydration will not only prevent negative side effects from supplements, but it will help you perform optimally in the gym and improve the way you feel. Some will experience a little water weight from creatine, but this is only temporary and any water weight that occurs will dissipate when you finish a cycle of creatine.
Ideally, creatine should be cycled to prevent the body from adapting to it's use and help it maintain effectiveness. As a rule of thumb, it is recommended to cycle creatine: 12 weeks on, followed by 4 weeks off. On your workout days, the dosage will depend on the product. Some products are designed to be taken before your workout (Con-Cret, Creatine Nitrate), while some are designed to be taken post workout (CreaPure). On rest days, take them on an empty stomach whenever it is convenient for you.
When using some creatines, it is also recommended to have a loading phase. This means that you use a higher dosage of the product for the first week or so to saturate the levels of creatine in your system before lowering it down to the normal 'maintenance' level. Once again, follow the directions of the label of your product.
There are also many varieties of creatine on the market today, each with it's own proposed benefits that make it better than the next. Among these are CreaPure (monohydrate), ethyl ester, Kre-Alkalyn, Hydrochloride, Chelate, and Nitrate.