Creatine is probably the best known and widely used sport supplement, for the simple fact that it's had hundreds of studies showing that it works. Creatine occurs naturally in our bodies, its main function is the regeneration of ATP (Adenosine Triphosphate), which is the main fuel source for working muscles. ATP muscle stores only last about 10 seconds and so supplementing with creatine increases this storage to result in longer lasting, intense workouts.
Supplementing with creatine helps to increase strength and builds lean muscle mass during intense short duration exercise, such as weight training. Creatine helps to increase recovery times, both during and after workouts. The ability to work out for longer, with more intensity, has an anabolic effect which leads to new muscle mass. Creatine also has a cell volumising effect, which means that you'll feel more pumped due to the extra water that's pulled into the muscle cell.