What is Creatine? 
Creatine is a compound that can be made in our bodies or taken as a dietary supplement. The chemical name for Creatine is methyl guanidine-acetic acid. That sure is a mouth full - which is why it is much easier to just call it creatine. Here is the chemical makeup of creatine -

Creatine is made up of three amino acids - Arginine, Glycine and Methionine. Our liver has the ability to combine these three amino acids and make creatine. The other way we get creatine is from our diet.
How much Creatine do we have in our body?
This varies based on the amount of muscle mass you have and your weight. On average a 160 pound person would have about 120 grams of creatine stored in their body.
Where is Creatine stored in our body?
It is believed that 95 - 98% of the creatine in our body is stored in our muscles. The remaining 2 - 5% is stored in various other parts of the body including the brain, heart and testes.
So what does creatine do?
Now is when the fun begins. First, before we answer this question - understand that the theory of what creatine does - is just that - theory. It is amazing how little we actually know about what goes on in our body. Anyway, we will outline what the majority of research currently agrees on in terms of what role creatine plays in our body.
1. Provide additional energy for your muscles
Time for a quick and simple biology lesson. In your body you have a compound called ATP (adenosine tri-phosphate). Think of ATP as an energy containing compound. What is important to know about ATP is that the body can very quickly get energy from a ATP reaction. You have other sources of energy such as carbohydrates and fat - but they take longer to convert into a useable energy source. When you are doing an intense quick burst activity - such as lifting a weight or sprinting, your muscles must contract and need a quick source of energy. This immediate energy comes from ATP.
Okay - still with us? Here is where it gets interesting. When your muscles use ATP for energy a chemical process happens where the ATP is broken down into two simpler chemicals ADP (adenosine di-phosphate) and inorganic phosphate. This process of ATP turning into ADP releases the energy which gives your muscles the ability to contract. Unfortunately, we do not have an endless supply of ATP. In fact, your muscles only contain enough ATP to last about 10-15 seconds at maximum exertion. In case you were wondering - no, the ADP can not be used to create more energy for your muscles.
Here is where the creatine comes in - or more specifically the creatine phosphate (CP). We don't want to go into great detail on creatine vs. creatine phosphate now (that is in a later article) - all you need to know now is that the majority of creatine that is stored in the muscles bonds with abundant phosphorus stores in the muscles and is converted into Creatine Phosphate (CP). CP is able to react with the ADP in your body and turn "useless" ADP back into the "super useful" energy source - ATP. More ATP in your body means more fuel for your muscles.
2. Volumization of your muscles
Looks like we just made up that word -Volumization - doesn't it? Actually, it's just a fancy name for the process of pulling fluid into the muscle cells and thus increasing the volume of the muscles. Creatine has been shown to pull water into your muscle cells, which increases the size of your muscles. Don't get to excited - it is not clear how great an effect this has. Point #1 is a much clearer benefit of creatine.
3. Buffer Lactic Acid build-up
New research has shown that creatine can help buffer lactic acid that builds-up in the muscles during exercise. This leads to that nasty burning feel you get in your muscles. Scientifically it is a complicated process - basically the creatine bonds with a Hydrogen ion and that helps delay the build up of lactic acid. More research needs to be done to see if this point is true.
4. Enhances Protein Synthesis
There is some data to indicate that creatine helps put the body in a more anabolic state where protein synthesis can occur. The more protein synthesis - the greater the muscle gain.
Well - there you have what creatine does in a very simplified nutshell. Of all 4 points - point #1 is the most use of creatine in the body. The other points are more debated - but still look to be valid.
Is the 120 grams of creatine in my body enough?
Maybe. The whole idea behind taking creatine as supplement is that if you workout you burn-up a lot of creatine. If you take a creatine supplement you will have more energy - because the ATP energy cycle can go on for a longer time. We go into supplementation in another article - but here is the quick run down. Unfortunately your muscle's creatine supply is not limitless. The average human has between 3.5 and 4 grams of creatine per kilogram of muscle. Once you use up the creatine in your muscle you have to rest your muscles and wait a while before you can exercise the muscle again. Studies have shown that the human muscle can store up to 5 grams of creatine per kilogram. So, by taking a creatine supplement you can raise your levels from 3.5 to 5 grams of creatine - and thus enjoy more of the benefits of creatine.