I recently went through a rigorous training for a law enforcement academy. I started taking creatine a few weeks prior, because we had to pass a PT test. It made a difference with endurance. I could go longer without the burn. The creatine acts as a buffer against lactic acid which causes the burn when you are working out. You have to take it about 4 weeks before you notice a difference. It increases the fluid in your muscles, so make sure you drink plenty of water to stay hydrated.

the previous responder is pretty right on, except for one part. "creatine" is actually creatine phosphate. it is the breaking of the bond between the creatine and the phosphate which will provide you with the energy boost and require the water.