Creatine is a naturally occurring compound in the body that helps provide energy to muscles. It is also commonly used as a supplement to boost physical performance during exercise or athletic activities. Full definition
I will leave out the muscle gains / endurance / strength increase part of creatine out of this article as I believe a lot has been written on the topic already. (healthyeatingharbor.com)
There are possible safety concerns with creatine supplementation that will be discussed later. (muscle-fitness-training.com)
A review of creatine supplementation in age - related diseases: more than a supplement for athletes. (athleanx.com)